Add opt-in --remove-unused-type-ignores flag

[nitishagar]

Summary

pyrefly suppress --remove-unused and pyrefly check --remove-unused-ignores remove unused # pyrefly: ignore comments, but they leave unused # type: ignore comments behind — even though Pyrefly honors # type: ignore and already detects when one is unused. After cleaning up suppressions, any stale # type: ignore comments have to be found and deleted by hand, which is painful in codebases that adopted # type: ignore (e.g. coming from another type checker).

This adds an opt-in --remove-unused-type-ignores flag (on both check and suppress) that additionally removes unused # type: ignore comments, working just like the existing --remove-unused / --remove-unused-ignores flags. It is off by default, so # type: ignore comments — which are shared with other type checkers — are never touched unless the flag is explicitly passed.

The change threads a remove_type_ignores boolean into the existing removal path and adds a single gated arm that handles the UnusedTypeIgnore errors Pyrefly already produces; unused detection is unchanged.

$ pyrefly check --remove-unused-ignores --remove-unused-type-ignores repro.py
# removes both "# pyrefly: ignore" and unused "# type: ignore"
$ pyrefly check --remove-unused-ignores repro.py
# removes only "# pyrefly: ignore" (default, unchanged)

Fixes #3984.

Test plan

• cargo build -p pyrefly
• cargo test -p pyrefly remove_unused — 23 passed, including new test_remove_unused_type_ignore_inline / test_remove_unused_type_ignore_with_code and a regression test asserting the comment is kept when the flag is absent.
• cargo clippy -p pyrefly --tests
• Manual repro from the issue: with the flag both comments are removed; without it only the pyrefly one is removed.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅