Remove unused pngmath sphinx extension
Remove unused pngmath sphinx extension

The extension is not used, remove it.

Change-Id: I3ea4b529025c8ac3c4092f3720124647861e6668

stevelle/glance,openstack/glance,rajalokan/glance,rajalokan/glance,openstack/glance,openstack/glance,klmitch/glance,klmitch/glance,stevelle/glance