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/571721

Change-Id: I40eb85cae6027fe188264807212d41d9737f7452

.gitignore

@@ -27,7 +27,6 @@ develop-eggs
 # Other
 *.DS_Store
 .stestr
-.testrepository
 .tox
 .idea
 .venv

lower-constraints.txt

@@ -92,7 +92,6 @@ sphinxcontrib-websupport==1.0.1
 stestr==1.0.0
 stevedore==1.20.0
 tempest==17.1.0
-testrepository==0.0.18
 testtools==2.2.0
 traceback2==1.4.0
 unittest2==1.1.0