LP#1681864: Add pocommentclean for db.seed cleanup prior to updatepo

LP#1681864: Add pocommentclean for db.seed cleanup prior to updatepo

By adding a segment to run pocommentclean against the db.seed locale file
prior to doing updatepo against the file from the PO template, we clear
any unwanted old and outdated comment IDs prior to applying clean newly
synchronized comment IDs from the PO template.

Signed-off-by: Ben Shum <ben@evergreener.net>