Disable sphinxcontrib.napoleon, seems unused and isn't packaged in Debian

--- a/docs/source/conf.py
+++ b/docs/source/conf.py
@@ -24,7 +24,6 @@
 # ones.
 extensions = [
     'sphinx.ext.autodoc',
-    'sphinx.ext.napoleon',
 ]
 
 # Add any paths that contain templates here, relative to this directory.
