remove debug extras from setup.py

[esquires]

pip install 'ray[debug]' ceased installing extra packages in
0826f95

Why are these changes needed?

Documentation indicates pip install 'ray[debug]' is recommended but [debug] does not install any extra packages.

Related issue number

#7031

See also the discussion here

Checks

• [x ] I've run scripts/format.sh to lint the changes in this PR.
• [x ] I've included any doc changes needed for https://docs.ray.io/en/master/.
• [x ] 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
  • [x ] This PR is not tested :(