gfs2: Rename gfs2_trans_{add_unrevoke => remove_revoke}

Rename gfs2_trans_add_unrevoke to gfs2_trans_remove_revoke: there is no
such thing as an "unrevoke" object; all this function does is remove
existing revoke objects plus some bookkeeping.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>