schema CHANGE remove private_destructor parameter from ly_ctx_destroy()

The mechanism was not reliable and not used often. libyang provide
function to traverse the schema and free the set data manually or the
caller can maintain some information to free the set data directly.

This commit changes API.
diff --git a/tests/utests/basic/test_context.c b/tests/utests/basic/test_context.c
index a66dce4..6544372 100644
--- a/tests/utests/basic/test_context.c
+++ b/tests/utests/basic/test_context.c
@@ -99,7 +99,7 @@
     assert_int_equal(LY_SUCCESS, ly_ctx_unset_searchdir(UTEST_LYCTX, NULL));
 
     /* cleanup */
-    ly_ctx_destroy(UTEST_LYCTX, NULL);
+    ly_ctx_destroy(UTEST_LYCTX);
 
     /* test searchdir list in ly_ctx_new() */
     assert_int_equal(LY_EINVAL, ly_ctx_new("/nonexistingfile", 0, &UTEST_LYCTX));
@@ -114,7 +114,7 @@
 test_options(void **state)
 {
     /* use own context with extra flags */
-    ly_ctx_destroy(UTEST_LYCTX, NULL);
+    ly_ctx_destroy(UTEST_LYCTX);
 
     assert_int_equal(LY_SUCCESS, ly_ctx_new(NULL, 0xffff, &UTEST_LYCTX));
 
@@ -205,7 +205,7 @@
     struct lys_glob_unres unres = {0};
 
     /* use own context with extra flags */
-    ly_ctx_destroy(UTEST_LYCTX, NULL);
+    ly_ctx_destroy(UTEST_LYCTX);
 
     /* invalid arguments */
     assert_int_equal(0, ly_ctx_get_change_count(NULL));
@@ -309,7 +309,7 @@
     const struct lys_module *mod1, *mod2, *import;
 
     /* use own context with extra flags */
-    ly_ctx_destroy(UTEST_LYCTX, NULL);
+    ly_ctx_destroy(UTEST_LYCTX);
 
     /* import callback provides newer revision of module 'a' than present in context, so when importing 'a', the newer revision
      * from the callback should be loaded into the context and used as an import */
@@ -328,7 +328,7 @@
     assert_string_equal("2019-09-17", import->revision);
     assert_int_equal(0, import->implemented);
     assert_non_null(ly_ctx_get_module(UTEST_LYCTX, "a", "2019-09-16"));
-    ly_ctx_destroy(UTEST_LYCTX, NULL);
+    ly_ctx_destroy(UTEST_LYCTX);
 
     /* import callback provides older revision of module 'a' than present in context, so when importing a, the newer revision
      * already present in the context should be selected and the callback's revision should not be loaded into the context */
@@ -660,7 +660,7 @@
     assert_int_equal(LY_SUCCESS, ly_ctx_new_ylmem(TESTS_SRC "/modules/yang/", yanglibrary_only, LYD_XML, 0, &ctx_test));
     assert_ptr_not_equal(NULL, ly_ctx_get_module(ctx_test, "ietf-yang-library", "2019-01-04"));
     assert_null(ly_ctx_get_module(ctx_test, "ietf-netconf", "2011-06-01"));
-    ly_ctx_destroy(ctx_test, NULL);
+    ly_ctx_destroy(ctx_test);
 
     /* test loading module, should also import other module */
     assert_int_equal(LY_SUCCESS, ly_ctx_new_ylmem(TESTS_SRC "/modules/yang/", with_netconf, LYD_XML, 0, &ctx_test));
@@ -669,14 +669,14 @@
     assert_int_not_equal(NULL, ly_ctx_get_module(ctx_test, "ietf-netconf-acm", "2018-02-14"));
     assert_int_equal(0, ly_ctx_get_module(ctx_test, "ietf-netconf-acm", "2018-02-14")->implemented);
     assert_int_equal(LY_ENOT, lys_feature_value(ly_ctx_get_module(ctx_test, "ietf-netconf", "2011-06-01"), "url"));
-    ly_ctx_destroy(ctx_test, NULL);
+    ly_ctx_destroy(ctx_test);
 
     /* test loading module with feature if they are present */
     assert_int_equal(LY_SUCCESS, ly_ctx_new_ylmem(TESTS_SRC "/modules/yang/", with_netconf_features, LYD_XML, 0, &ctx_test));
     assert_ptr_not_equal(NULL, ly_ctx_get_module(ctx_test, "ietf-netconf", "2011-06-01"));
     assert_ptr_not_equal(NULL, ly_ctx_get_module(ctx_test, "ietf-netconf-acm", "2018-02-14"));
     assert_int_equal(LY_SUCCESS, lys_feature_value(ly_ctx_get_module(ctx_test, "ietf-netconf", "2011-06-01"), "url"));
-    ly_ctx_destroy(ctx_test, NULL);
+    ly_ctx_destroy(ctx_test);
 
     /* test with not matching revision */
     assert_int_equal(LY_EINVAL, ly_ctx_new_ylmem(TESTS_SRC "/modules/yang/", garbage_revision, LYD_XML, 0, &ctx_test));
@@ -687,7 +687,7 @@
     /* test creating without ietf-yang-library */
     assert_int_equal(LY_SUCCESS, ly_ctx_new_ylmem(TESTS_SRC "/modules/yang/", no_yanglibrary, LYD_XML, LY_CTX_NO_YANGLIBRARY, &ctx_test));
     assert_int_equal(NULL, ly_ctx_get_module(ctx_test, "ietf-yang-library", "2019-01-04"));
-    ly_ctx_destroy(ctx_test, NULL);
+    ly_ctx_destroy(ctx_test);
 }
 
 static LY_ERR
@@ -815,7 +815,7 @@
             "}\n";
 
     /* use own context with extra flags */
-    ly_ctx_destroy(UTEST_LYCTX, NULL);
+    ly_ctx_destroy(UTEST_LYCTX);
     const char *feats[] = {"f1", NULL};
     assert_int_equal(LY_SUCCESS, ly_ctx_new(NULL, LY_CTX_SET_PRIV_PARSED, &UTEST_LYCTX));
     UTEST_ADD_MODULE(schema_a, LYS_IN_YANG, feats, NULL);