Remove the unused coding style modules

Python modules related to coding style checks (listed in blacklist.txt in
openstack/requirements repo) are dropped from lower-constraints.txt
as they are not actually used in tests (other than pep8).

more info: https://github.com/openstack/requirements/blob/master/blacklist.txt

Change-Id: Iac09dcd4c97950717c72f2f6a669b2f11e5f6ee6

lower-constraints.txt

@@ -17,12 +17,10 @@ eventlet==0.18.2
 extras==1.0.0
 fasteners==0.7.0
 fixtures==3.0.0
-flake8==2.5.5
 futurist==1.2.0
 gitdb==0.6.4
 GitPython==1.0.1
 greenlet==0.4.10
-hacking==0.12.0
 imagesize==0.7.1
 iso8601==0.1.11
 Jinja2==2.10
@@ -30,7 +28,6 @@ keystoneauth1==3.4.0
 kombu==4.6.6
 linecache2==1.0.0
 MarkupSafe==1.0
-mccabe==0.2.1
 mock==2.0.0
 mox3==0.20.0
 msgpack-python==0.4.0
@@ -51,11 +48,9 @@ oslotest==3.2.0
 Paste==2.0.2
 PasteDeploy==1.5.0
 pbr==2.0.0
-pep8==1.5.7
 pifpaf==2.2.0
 prettytable==0.7.2
 pycparser==2.18
-pyflakes==0.8.1
 Pygments==2.2.0
 pyinotify==0.9.6
 pyngus==2.2.0