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: I06fec95c12a239a6d143a8bf7d6e072d4eeb6d2d
5d217ffea6