Remove unused fake_flags

fake_flags.py should have been removed in change
I3424d0a401b3ef7a3254d3e913263554361a52ff but somehow it's left
untouched in that change.  This module has been replaced by
cinder/tests/conf_fixture.py, and shouldn't be around anymore.

Change-Id: Ied840b95253d998c1b3326c1dde9d469fa783c85
Merge "Remove unused fake_flags"