[red-knot] Remove unneeded branch in Type::is_equivalent_to()

[AlexWaygood]

Summary

We understand sys.version_info branches now! As such, I believe this branch is no longer required; all tests pass without it. I also ran QUICKCHECK_TESTS=100000 cargo test -p red_knot_python_semantic -- --ignored types::property_tests::stable, and no tests failed except for the known issue with Type::is_assignable_to() (#14899)

Test Plan

See above

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[sharkdp]

I removed an earlier version of this check in my statically-known-branches PR, but probably missed this during a rebase. The early-return-version of is_equivalent_to arrived at some point when that PR was open, IIRC. Anyway, thank you!