[tune] remove scikit-optimize search algorithm

[matthewdeng]

Why are these changes needed?

Removing SkOptSearch as there is little usage, and the scikit-optimize library has not been maintained.

1. The last commit is from 3 years ago.
2. The latest stable release does not support Python 3.11.
3. The repo has now been archived.

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 :(

[can-anyscale]

some doc tests are failing; otherwise ci change looks great, thanks for doing this matt

[justinvyu]

Could we add a bit more of explanation for removing in the PR description (library is not maintained, 3.11 support, etc.)? Thanks!

[woshiyyya]

LGTM. Any other searchers to deprecate?

[angelinalg]

Love the purging and maintenance! Thank you!