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: I1c776ee7e460b60b313a6a7023e62a4d8bfa1be0

lower-constraints.txt

@@ -26,9 +26,7 @@ dulwich==0.19.0
 extras==1.0.0
 fasteners==0.14.1
 fixtures==3.0.0
-flake8==2.5.5
 futurist==1.6.0
-hacking==0.12.0
 horizon==17.1.0
 idna==2.6
 imagesize==1.0.0
@@ -41,7 +39,6 @@ jsonschema==2.6.0
 keystoneauth1==3.4.0
 linecache2==1.0.0
 MarkupSafe==1.0
-mccabe==0.2.1
 mock==2.0.0
 monotonic==1.4
 msgpack==0.5.6
@@ -62,11 +59,9 @@ oslo.utils==3.36.0
 osprofiler==2.3.0
 packaging==17.1
 pbr==3.1.1
-pep8==1.5.7
 Pint==0.8.1
 prettytable==0.7.2
 pycparser==2.18
-pyflakes==0.8.1
 Pygments==2.2.0
 pymongo==3.6.1
 pyOpenSSL==17.5.0