Drop relabelto_domain() macro and its associated definitions.

This was originally to limit the ability to relabel files to
particular types given the ability of all domains to relabelfrom
unlabeled files.  Since the latter was removed by
Ied84f8b4b1a0896c1b9f7d783b7463ce09d4807b, this no longer serves
any purpose.

Change-Id: Ic41e94437188183f15ed8b3732c6cd5918da3397
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>