[3.x] GDScript: Don't warn about RETURN_VALUE_DISCARDED by default

[SysError99]

Following #69002. Turns out 4.x has a lot of changes in the GDScript module so cherry-picking it directly is out of question.

I can't really see the reason to keep it enabled by default even with 3.x especially when more of new features cherry-picked from 4.x introduce a lot of discarded values. Anyway, this left to be justified by the maintainer since warnings in 3.x aren't apparent in its own IDE but it's very annoying for users that use external editors.

[AThousandShips]

Please use git rebase instead of merging into your branch, it make the git history very muddled and makes it harder to squash your commits

[AThousandShips]

You accidentally re-signed all those commits instead of dropping them, you will need to remove all but your own contributions

[SysError99]

Thanks, it's all my bad since my knowledge is still very limited. I'll redo everything now since this PR is pretty much broken.

[AThousandShips]

Will probably be easiest, make sure to not create your PR from your 3.x branch but a dedicated PR branch