Drop pngmath extenstion from sphinx config

Bug: https://bugs.gentoo.org/594626

--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -74,7 +74,6 @@ def cd(newpath):
     'sphinx.ext.intersphinx',
     'sphinx.ext.mathjax',
     'sphinx.ext.napoleon',
-    'sphinx.ext.pngmath',
     'sphinx.ext.todo',
     'sphinx.ext.viewcode',
     #'sphinxcontrib.bibtex',