Remove intersphinx mappings

None of our documentation references any external sphinx documents so we
don't need the intersphinx mappings.

This started as a rename from the old glance.openstack.org site to the
newer docs.openstack.org/developer/glance however they may as well be
removed if not used.

Closes-Bug: #1353817
Change-Id: I638c4eeab7d01ec2f26a0d9d532f0006f4f75e72