bpo-43244: Remove the PyAST_Validate() function (GH-24911)

Remove the PyAST_Validate() function. It is no longer possible to
build a AST object (mod_ty type) with the public C API. The function
was already excluded from the limited C API (PEP 384).

Rename PyAST_Validate() function to _PyAST_Validate(), move it to the
internal C API, and don't export it anymore (replace PyAPI_FUNC with
extern).

The function was added in bpo-12575 by
the commit 832bfe2ebd5ecfa92031cd40c8b41835ba90487f.
diff --git a/Parser/pegen.c b/Parser/pegen.c
index 84bdf8d..24aa3af 100644
--- a/Parser/pegen.c
+++ b/Parser/pegen.c
@@ -1,4 +1,5 @@
 #include <Python.h>
+#include "pycore_ast.h"           // _PyAST_Validate()
 #include <errcode.h>
 #include "tokenizer.h"
 
@@ -1271,7 +1272,7 @@
         p->start_rule == Py_file_input ||
         p->start_rule == Py_eval_input)
     {
-        if (!PyAST_Validate(res)) {
+        if (!_PyAST_Validate(res)) {
             return NULL;
         }
     }