audit: drop the meaningless and format breaking word 'user'

userspace audit messages look like so:

type=USER msg=audit(1271170549.415:24710): user pid=14722 uid=0 auid=500 ses=1 subj=unconfined_u:unconfined_r:auditctl_t:s0-s0:c0.c1023 msg=''

That third field just says 'user'.  That's useless and doesn't follow the
key=value pair we are trying to enforce.  We already know it came from the
user based on the record type.  Kill that word.  Die.

Signed-off-by: Eric Paris <eparis@redhat.com>