remove deprecated modules

[st--]

Why are these changes needed?

#36984 removed warn_structure_refactor, but failed to remove the ray.air.callbacks submodules that still rely on it. While arguably in my specific case the presence of their remainder helped me update some old code, it would be cleaner if ray failed with an ImportError, making it clear that it's an issue on the user side, rather than something that looks like it's a bug.

Related issue number

Closes #45103

Checks

This PR only removes no-longer-used modules, and does not add any new code.

• 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 :(

[matthewdeng]

Thanks for catching and fixing this!