tox: Stop linting as part of docs target
tox: Stop linting as part of docs target

This linting makes building docs an even more painful process than it
would otherwise be. We already do this as part of the pep8 target,
which the gate runs, so there's no need to do this here.

Change-Id: I36ae872dd21299ad5d165422cb83564eafd89bea
Signed-off-by: Stephen Finucane <4725dc7f3632bf126d9fbdc30c347e91e8d2fe16@redhat.com>

openstack/nova,mahak/nova,mahak/nova,mahak/nova,openstack/nova,openstack/nova