chore: partial revert of #1297

[parthea]

#1297 included an incorrect change in setup.py that was missed during code review.

The following error appears without the fix in this PR

python-pubsub# pip install .
Processing /python-pubsub
  Preparing metadata (setup.py) ... error
  error: subprocess-exited-with-error
  
  × python setup.py egg_info did not run successfully.
  │ exit code: 1
  ╰─> [1 lines of output]
      error in google-cloud-pubsub setup command: 'extras_require' must be a dictionary whose values are strings or lists of strings containing valid project/version requirement specifiers.
      [end of output]

[ohmayr]

I suspect the pip command to fail without the extra.