requirements: Remove explicit pbr dependency
requirements: Remove explicit pbr dependency

We don't use this at runtime. No need to install it.

Change-Id: I5d7d526beebc39de2fa0ff8c6b1d2458c6a3d0bb
Signed-off-by: Stephen Finucane <492121341a95b3c3aab646bed44634f739dd019b@redhat.com>

openstack/oslo.utils