RevisionStore: Remove reference to unimplemented 'ar' blob address schema

RevisionStore: Remove reference to unimplemented 'ar' blob address schema

It was thought that an 'ar:' schema for SqlBlobStore addresses would be
needed to support archive rows from before MediaWiki 1.5 that still
stored the text in ar_text and ar_flags instead of using the text table
and ar_text_id. But this schema was never actually implemented.

Now I5608c6b6 has migrated that content to the text table and
ar_text_id, and I18f1c740b will remove the database fields entirely. So
there's no reason to ever implement this schema.

Change-Id: Ic449478c244bb13a8c5139e20488e876c0a793bb