Remove unused pngmath Sphinx extension
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

openstack/poppy,openstack/poppy,stackforge/poppy,stackforge/poppy,openstack/poppy,stackforge/poppy