don't ignore rules that are already enforced

H302 / H304 are clean in the neutron source tree, so it seems
like they shouldn't be ignored. This requires no code changes to
enable.

If there is a longer standing reason for these ignores, would be
great to document in tox.ini.

Change-Id: Ib0f483e4f8fbd328815775a2a6f58be448172bd9
Merge "don't ignore rules that are already enforced"