docs: drop sphinxmark

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: Ia273b2a4ec77e693ee9e523ffbbb193c128b0410

doc/requirements.txt

@@ -6,5 +6,4 @@
 sphinx!=1.6.6,>=1.6.2 # BSD
 openstackdocstheme>=1.18.1 # Apache-2.0
 reno>=2.5.0 # Apache-2.0
-sphinxmark>=0.1.14 # Apache-2.0
 doc8>=0.6.0 # Apache-2.0

doc/source/conf.py

@@ -23,7 +23,6 @@
 # serve to show the default.
 
 import pbr.version
-import os
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
@@ -41,7 +40,6 @@ import os
 extensions = [
     'openstackdocstheme',
     'sphinx.ext.autodoc',
-    'sphinxmark'
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -312,17 +310,3 @@ texinfo_documents = [
 
 # If true, do not generate a @detailmenu in the "Top" node's menu.
 # texinfo_no_detailmenu = False
-
-
-watermark = os.popen("git branch --contains $(git rev-parse HEAD)\
-| awk -F/ '/stable/ {print $2}'").read().strip(' \n\t').capitalize()
-if watermark == "":
-    watermark = "Pre-release"
-
-# -- Options for sphinxmark -----------------------------------------------
-sphinxmark_enable = True
-sphinxmark_div = 'docs-body'
-sphinxmark_image = 'text'
-sphinxmark_text = watermark
-sphinxmark_text_color = (128, 128, 128)
-sphinxmark_text_size = 70