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/plugins_exts_yangdata.c b/src/plugins_exts_yangdata.c
index 4978f35..75c4e57 100644
--- a/src/plugins_exts_yangdata.c
+++ b/src/plugins_exts_yangdata.c
@@ -52,10 +52,10 @@
     uint32_t prev_options = cctx->options;
 
     /* yang-data can appear only at the top level of a YANG module or submodule */
-    if (c_ext->parent_type != LYEXT_PAR_MODULE) {
+    if ((c_ext->parent_stmt != LY_STMT_MODULE) && (c_ext->parent_stmt != LY_STMT_SUBMODULE)) {
         lyext_log(c_ext, LY_LLWRN, 0, cctx->path,
                 "Extension %s is ignored since it appears as a non top-level statement in \"%s\" statement.",
-                p_ext->name, lyext_parent2str(c_ext->parent_type));
+                p_ext->name, ly_stmt2str(c_ext->parent_stmt));
         return LY_ENOT;
     }
     /* check mandatory argument */