Remove testrepository

This commit is a follow-up of "switch to using stestr"
which was merged already [1].
After switch to using stestr, testrepository is unnecessary
and should be removed.

[1] https://review.openstack.org/578521

Change-Id: Ic4005aa6f4bf5fb52fc207ff1cec666ccef8d0da

.gitignore

@@ -11,7 +11,6 @@ cover
 .coverage
 coverage.xml
 .stestr
-.testrepository
 imagebuild/.image_cache
 imagebuild/coreos/build
 imagebuild/coreos/dist

lower-constraints.txt

@@ -87,7 +87,6 @@ sphinxcontrib-pecanwsme==0.8.0
 sphinxcontrib-websupport==1.0.1
 stestr==1.0.0
 stevedore==1.20.0
-testrepository==0.0.20
 testtools==2.2.0
 traceback2==1.4.0
 unittest2==1.1.0