Remove openstackdocstheme from test-requirements.txt file

The openstackdocstheme is included by ``doc/requirements.txt``. We
move all doc requirements to ``doc/requirements.txt``.

Change-Id: I3e3acf07826a496b3d8514941afa908624fb74f4

test-requirements.txt

@@ -8,7 +8,6 @@ coverage!=4.4,>=4.0 # Apache-2.0
 doc8>=0.6.0 # Apache-2.0
 extras>=1.0.0 # MIT
 hacking>=0.10.0,<1.1.0
-openstackdocstheme>=1.18.1 # Apache-2.0
 oslo.log>=3.36.0 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 PrettyTable<0.8,>=0.7.1 # BSD