Remove unused pngmath Sphinx extension

There's no RST file that uses ".. math" and thus
the pngmath Sphinx extension is not used and can
get removed.

Change-Id: I3ea4b529025c8ac3c4092f3720124647861e6668

Author: ajaeger

doc/source/conf.py

@@ -29,7 +29,7 @@
 
 # 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.todo', 'sphinx.ext.pngmath']
+extensions = ['sphinx.ext.autodoc', 'sphinx.ext.todo']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']