Remove strutils and timeutils from openstack-common.conf

Since Keystone is changed to use strutils and timeutils from
oslo.utils, it doesn't directly use these modules from
oslo-incubator anymore, as such, they're removed from
openstack-common.conf.

Since these modules are still used internally by oslo-incubator
modules, they aren't removed from keystone in a sync.

Change-Id: Ia31e455c6441cfbfbe33271ccdef1030a8c3d5cc