chore(apt): remove unused apt key listing code

[wak327]

Proposed Commit Message

chore(apt): remove unused apt key listing code

apt_key() implemented 'list' and 'finger' commands next to 'add', but
nothing calls them: add_apt_key_raw() is the only caller of apt_key()
and it only ever adds keys. Rather than fix the listing logic, which
assumes every file under /etc/apt/trusted.gpg.d/ is a keyring, drop it
together with GPG.list_keys(), which only existed to serve it.

Fixes GH-5333

Additional Context

Fixes #5333

This follows the suggestion in GH-5333 to delete the code rather than fix it. The only call site of apt_key() passes "add", and nothing else uses GPG.list_keys(). APT_LOCAL_KEYS stays because tests/integration_tests/modules/test_apt_functionality.py still uses it.

apt_key() now raises ValueError for any command other than "add", and a new test covers this. test_kill_gpg_succeeds only called list_keys() to "start" gpg, so it now calls dearmor() for that instead.

Verified with the pinned black, isort, ruff, pylint and mypy, and the full unit suite passes (5,775 tests).

Test Steps

tox -e py3 -- tests/unittests/config/test_apt_key.py tests/unittests/test_gpg.py tests/unittests/config/

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)