Remove mhealpy use

gwcelery/tasks/external_skymaps.py

@@ -10,7 +10,6 @@ from ligo.skymap.io import fits, read_sky_map, write_sky_map
 import gcn
 import healpy as hp
 import lxml.etree
-from mhealpy import HealpixMap
 import re
 import ssl
 import urllib