Delete openstack.common.context
Delete openstack.common.context

This file is not being used, and actually it seems that we've been
relying on a custom context library all along. So I'm deleting this
for now, and a subsequent patch will actually start using oslo_context
instead.

Change-Id: I769fe814db95d22c5ceb20c2cbfbf870dd480e7d

openstack/barbican,cloudkeep/barbican,cneill/barbican,cneill/barbican,cloudkeep/barbican,MCDong/barbican,openstack/barbican,MCDong/barbican