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: Ib13b99821aef29a78a4d83fd94991379bf9d4294

doc/requirements.txt

@@ -8,4 +8,3 @@ oslosphinx>=4.7.0 # Apache-2.0
 openstackdocstheme>=1.18.1 # Apache-2.0
 doc8>=0.6.0 # Apache-2.0
 reno>=2.5.0 # Apache-2.0
-sphinxmark>=0.1.14 # Apache-2.0

doc/source/conf.py

@@ -40,9 +40,8 @@ import os
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
 extensions = [
-    'sphinx.ext.autodoc',
-    'oslosphinx',
-    'sphinxmark'
+    'openstackdocstheme',
+    'sphinx.ext.autodoc'
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -320,17 +319,3 @@ 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