engine: Unassigned log type should not be logged

The Unassigned log type is used to ignore a logging
of an event. Hence, it should not appear in the resource
bundle.

This patch, back-ported from master branch resolves the reported issue.

Change-Id: I4dad0139cc65fe3c90f7c59cda5416db1a83c569
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1142859
Signed-off-by: Eli Mesika <emesika@redhat.com>
Downstream-Only