msgmerge keeps obsolete translation entries but we should filter them
out. Do the filtering with msgattrib.
Note this is only needed for log files, the other files are handled
correctly since update_catalog has an option for this.
This fixes the issue seen in glance:
https://review.openstack.org/#/c/105069/
Change-Id: I956e4af59d7bbd6e90583bdb9412c9bf85b87019