Remove requirements file for readthedocs.org

During the last release (63.0), readthedocs.org builds were failing,
complaining about the non existence of "requirements-docs.txt".

After escalating the problem, I decided to remove that option from the
configuration system.  The result was a successful build.  I can only
assume that readthedocs.org have everything we need.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

requirements-docs.txt

-# All pip installable requirements pinned for readthedocs.org
-fabric==1.10.0
-requests==1.2.3
-PyYAML==3.11
-Pillow==2.6.1
-aexpect==1.0.0
-# six is a stevedore depedency, but we also use it directly
-six==1.9.0
-# stevedore for loading "new style" plugins
-stevedore==1.20.0