GDScript: Don't warn about RETURN_VALUE_DISCARDED by default

[akien-mga]

This happens too often with normal usage of the API. The warning can still be useful to find actual bugs where discarding the return value wasn't intentional, but this should stay enabled manually, at least until we either improve the API to remove false positives, or improve the warning (e.g. to only warn about unused return value on const functions).

• Supersedes (for now) Remove the error code return from Object.connect. #58438.
• Reduces the need for Add capability to "filter" certain functions out of the warning linter that don't return a value that needs to be processed godot-proposals#682, but implementing that proposal or similar improvements would make it possible to re-enable this warning / improved version of it.

[adamscott]

Fine by me!

[mhilbrunner]

As discussed and agreed on in today's PR meeting

[clayjohn]

Thanks!