schema CHANGE remove LYEXT_PARENT duplicating ly_stmt

Removing the enumeration also leads to simplify the extension instance
structure where was the information about the parent structure duplicated.
diff --git a/src/parser_yang.c b/src/parser_yang.c
index 9649002..a3b5c03 100644
--- a/src/parser_yang.c
+++ b/src/parser_yang.c
@@ -862,8 +862,8 @@
     /* store the rest of information */
     e->format = LY_PREF_SCHEMA;
     e->prefix_data = ctx->parsed_mod;
-    e->insubstmt = insubstmt;
-    e->insubstmt_index = insubstmt_index;
+    e->parent_stmt = insubstmt;
+    e->parent_stmt_index = insubstmt_index;
 
     YANG_READ_SUBSTMT_FOR(ctx, kw, word, word_len, ret, ) {
         LY_CHECK_RET(parse_ext_substmt(ctx, kw, word, word_len, &e->child));