[Doc] drop obsolete sphinx.ext.pngmath

doc/sphinx/conf.py.in

@@ -24,7 +24,7 @@ sys.path = [os.path.abspath('@CMAKE_SOURCE_DIR@/src')]+sys.path
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ['sphinx.ext.autodoc', 'sphinx.ext.doctest', 'sphinx.ext.intersphinx', 'sphinx.ext.todo', 'sphinx.ext.pngmath']
+extensions = ['sphinx.ext.autodoc', 'sphinx.ext.doctest', 'sphinx.ext.intersphinx', 'sphinx.ext.todo']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']