[tune] remove test_client.py

[matthewdeng]

Why are these changes needed?

This test suite tests a bunch of Tune examples with Ray Client.

This PR removes these tests because:

1. The Ray Tune / Ray Client integration is not being maintained. The functionality is not documented nor is it expected to be "stable".
2. These tests are extremely flaky, which mostly seems to be due to timeouts.
3. The non-Client version of these examples are being tested in other test suites.

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(