Remove python-barbicanclient from test-requirements.txt

It seems that it was added back in Liberty where an effort was
started to setup functional tests, however it is not used at the
moment.

This patch removes it from the list of dependencies.  With time,
Tempest should handle all integration checks.

Change-Id: I9dddd470db9dc42a6fa7dbe138c0c6189d8afdd7