GDScript: Fix override signature check of script inheritance

Avoid treating the super class as a meta type for signature check, since
it is looking at the instance level for that.

modules/gdscript/tests/scripts/analyzer/features/inheritance_signature_check_no_meta.gd

@@ -0,0 +1,10 @@
+func test():
+	print("ok")
+
+# https://github.com/godotengine/godot/issues/71994
+class A:
+	extends RefCounted
+class B:
+	extends A
+	func duplicate():
+		pass

modules/gdscript/tests/scripts/analyzer/features/inheritance_signature_check_no_meta.out

@@ -0,0 +1,2 @@
+GDTEST_OK
+ok