types.rs: remove unused is_stdlib_symbol methods

[AlexWaygood]

Summary

These methods are now unused: we now instead use the KnownFunction, KnownClass and KnownInstance enums to track whether a FunctionLiteral, ClassLiteral or Instance type represents a specific symbol that we care about. I also don't believe that these methods are particularly efficient. Let's get rid of them; we can always add them back later if we rediscover a need for them.

Test Plan

cargo test -p red_knot_python_semantic

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[MichaReiser]

I'm now inclined to remove name and instead store the Symbolid....