Revert "GDScript: Don't allow built-in scripts to use class_name"

This reverts commit 42514bfd.

There is still an unsolved regression from this commit (#28002)
which needs to be addressed before cherry-picking this one.

modules/gdscript/gdscript_parser.cpp

@@ -3494,10 +3494,6 @@ void GDScriptParser::_parse_class(ClassNode *p_class) {
 					_set_error("'class_name' is only valid for the main class namespace.");
 					return;
 				}
-				if (self_path.empty()) {
-					_set_error("'class_name' not allowed in built-in scripts.");
-					return;
-				}
 				if (tokenizer->get_token(1) != GDScriptTokenizer::TK_IDENTIFIER) {
 
 					_set_error("'class_name' syntax: 'class_name <UniqueName>'");