Remove exclude coverage regex from coverage job

This commit removes the regex to exclude the coverage extension from
the tox coverage job. The regex was needed for the unit tests on the
coverage extension, (which would mock out coverage) however that
extension has since been removed from nova. So we no longer need to
exclude tests that don't exist anymore.

Change-Id: I45685b1113ecc78eda92f13755ff2274ff517829
Merge "Remove exclude coverage regex from coverage job"