Dependency of packaging is no longer needed

[allenhsu]

We use fpm to create in-house debs for all 3rd party libs, so we always check dependencies of 3rd party libs carefully. When doing the checks for python-api-core, we noticed that although packaging is included in setup.py, it's no longer used in the code base. I believe it can be safely removed. Not an issue, just a notice.

[parthea]

Hi @allenhsu,

Thank you for finding this! I agree we should remove it from setup.py and testing/constraints-3.6.txt . PRs are welcome!

[tseaver]

@parthea I don't think this can be a P1: no user is broken by this. In fact, I would mark it type: cleanup with no priority.

We also need to ensure that Owlbot wouldn't be restoring packaging anywhere.