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: I16752186e25ed505797c66f2fb8682e226b30900

doc/requirements.txt

@@ -6,5 +6,4 @@
 sphinx!=1.6.6,!=1.6.7,>=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

@@ -12,7 +12,6 @@
 # All configuration values have a default; values that are commented out
 # serve to show the default.
 
-import os
 import pbr.version
 
 # If extensions (or modules to document with autodoc) are in another directory,
@@ -31,7 +30,6 @@ import pbr.version
 extensions = [
     'openstackdocstheme',
     'sphinx.ext.autodoc',
-    'sphinxmark'
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -305,17 +303,3 @@ 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