Remove unused openstack.common.importutils

When the code in openstack.common.log was updated, it is now consuming the
importutils from oslo.utils. There are no longer local references to this
code.

Change-Id: I09ff72e307c1d5ee58388184a97a36ae183d7b0e
Partially-Implements: blueprint improve-oslo-usage
Merge "Remove unused openstack.common.importutils"