GDScript: fix regression when checking for virtual function implementation.

Unfortunately it appears the virtual function checks in #77324 are not
trustworthy at runtime, because objects can have scripts attached, but
this information is not always available at compile-time. These checks
need to be removed.

The rest of the PR is still useful, making all method flags available to
the analyzer, so a full revert isn't necessary/desirable.

This reopens #76938, which will need another solution.

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

@@ -1 +0,0 @@
-GDTEST_OK