Clean up deprecations in register_plugin

[crusaderky]

• Clean up deprecations introduced in Replace Client.register_plugin's idempotent argument with .idempotent attribute on plugins #8342
• Related: Clean up deprecations in SchedulerPlugin.remove_worker #9222

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    30 files  ±  0      30 suites  ±0   11h 5m 7s ⏱️ + 2m 10s
 4 103 tests  -  12   3 994 ✅  -  13    104 💤 ±0  5 ❌ +1 
59 575 runs   - 180  57 129 ✅  - 182  2 440 💤 ±0  6 ❌ +2 

For more details on these failures, see this check.

Results for commit 0fd7326. ± Comparison against base commit d9c0010.

This pull request removes 12 tests.

distributed.diagnostics.tests.test_nanny_plugin ‑ test_duck_typed_register_nanny_plugin_is_deprecated
distributed.diagnostics.tests.test_nanny_plugin ‑ test_register_plugin_with_idempotent_keyword_is_deprecated
distributed.diagnostics.tests.test_nanny_plugin ‑ test_register_worker_plugin_is_deprecated
distributed.diagnostics.tests.test_nanny_plugin ‑ test_register_worker_plugin_typing_over_nanny_keyword
distributed.diagnostics.tests.test_scheduler_plugin ‑ test_register_plugin_with_idempotent_keyword_is_deprecated
distributed.diagnostics.tests.test_scheduler_plugin ‑ test_register_scheduler_plugin_deprecated
distributed.diagnostics.tests.test_scheduler_plugin ‑ test_scheduler_plugin_in_register_worker_plugin_overrides
distributed.diagnostics.tests.test_scheduler_plugin ‑ test_scheduler_plugin_in_register_worker_plugin_overrides_nanny
distributed.diagnostics.tests.test_worker_plugin ‑ test_duck_typed_register_worker_plugin_is_deprecated
distributed.diagnostics.tests.test_worker_plugin ‑ test_register_plugin_with_idempotent_keyword_is_deprecated
…

♻️ This comment has been updated with latest results.

[crusaderky]

All failures are unrelated. Ready for review.