[flake8-pyi] Remove type parameter correctly when it is the last (PYI019)

[InSyncWithFoo]

Summary

Part of #15821.

Prior to this change, a PEP 695 type variable would not be removed correctly were it to be placed at the very end of the type parameter list:

# Original
def f[T, S](self: S) -> S: ...

# Before
def f[TS](self) -> Self: ...

# After
def f[T](self) -> Self: ...

Test Plan

cargo nextest run and cargo insta test.

[AlexWaygood]

Thank you!

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.