workflows: drop pybridge-c8s.yml

At some point this made sense as a very rudimentary smoke test that we
weren't using language features that weren't compatible with Python 3.6,
but we've long been running our entire unit tests suite under Python 3.6
in our tox workflow.

What's more: this workflow often fails due to inability to fetch patches
from the CentOS mirrors.

Let's drop this one.