sphinxmark is no longer compatible with the latest release of Sphinx
which is causing all of our documentation jobs to fail. This patch
removes it as our current usage of openstacktheme for documentation
already provides watermarks for current branch and notices for which
branch the documentation covers.
Change-Id: I4a2bcfd8ceaa0253a9f5368256dda9ed78e42be1
e49687c20d