Purge rep_barrier:seqnumDuringOpen on delete of Region

The Region GC Procedure only cleans the 'info' column family. We also write a rep_barrier column family as of HBASE-20115 . HBASE-20117 adds a chore to clean them up after-the-fact. I've not studied how rep_barrier works (There is a comment in MetaTableAccessor to add explaination).

This issue is about adding the deletion of the rep_barrier content on region delete (zhangduo will this mess up serial replication?).

I want to clean out these rows. They occasionally can be misinterpreted in such as the hbck report as 'Orphan Regions' or in simple loading tools, we'll find the rep_barrier row and then fail because no accompanying info:regioninfo.

Perhaps removing rep_barrier column family promptly is the wrong thing to do... we need the lag for replication to catch up.... Let me know zhangduo.

Here is what they look like:

hbase(main):050:0> get 'hbase:meta', 'XXXX,22d0e538,1572669183985.6aa8710020b8a4f9ea290539fc254a76.'
COLUMN                                                                                      CELL
 rep_barrier:seqnumDuringOpen                                                               timestamp=1573272944262, value=\x00\x00\x00\x00\x00\x00\x00\x02

They get updated on split and when location moves. I don't seem to be able to disable this facility – it is on always. It also called 'unused' in title of HBASE-20117.