Remove deprecated PubSub

[fjetter]

We flagged this as deprecated ~9 months ago and one of the unit tests is flaky so I suggest to pull the plug

[github-actions]

Unit Test Results

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

    27 files  +    13      27 suites  +13   11h 32m 46s ⏱️ + 7h 10m 26s
 4 100 tests  -      6   3 984 ✅  -      8    112 💤  -     1  4 ❌ +3 
51 400 runs  +28 163  49 087 ✅ +27 049  2 309 💤 +1 111  4 ❌ +3 

For more details on these failures, see this check.

Results for commit b82a574. ± Comparison against base commit 43e38b5.

This pull request removes 6 tests.

distributed.tests.test_pubsub ‑ test_basic
distributed.tests.test_pubsub ‑ test_client
distributed.tests.test_pubsub ‑ test_client_worker
distributed.tests.test_pubsub ‑ test_repr
distributed.tests.test_pubsub ‑ test_speed
distributed.tests.test_pubsub ‑ test_timeouts

♻️ This comment has been updated with latest results.

[hendrikmakait]

It looks like CI isn't happy and the extension is still imported in the client.

[jrbourbeau]

Thanks @fjetter. It'd be good to see if these are referenced over in dask/dask. I think they're probably in the docs at a minimum.

[fjetter]

I couldn't find any references to this in the dask/dask repo