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: I63d45aead0621c349590a03ace38bb29097c5f7d

lower-constraints.txt

@@ -10,10 +10,8 @@ docutils==0.14
 dulwich==0.19.0
 extras==1.0.0
 fixtures==3.0.0
-flake8==2.5.5
 gitdb2==2.0.3
 GitPython==2.1.8
-hacking==2.0.0
 idna==2.6
 imagesize==1.0.0
 iso8601==0.1.12
@@ -21,7 +19,6 @@ Jinja2==2.10
 keystoneauth1==3.4.0
 linecache2==1.0.0
 MarkupSafe==1.0
-mccabe==0.2.1
 mox3==0.25.0
 msgpack==0.5.6
 netaddr==0.7.19
@@ -38,8 +35,6 @@ oslotest==3.3.0
 packaging==17.1
 Parsley==1.3
 pbr==3.1.1
-pep8==1.5.7
-pyflakes==0.8.1
 Pygments==2.2.0
 pyinotify==0.9.6
 pyparsing==2.2.0