MB-8460 kill leftover _replicator database

- we assume that if the cluster is not provisioned
  and _replicator database exists and contains more
  than 0 records, then this database is a leftover
  from some previous installation that had the same
  directory setup for its data files.
- to prevent crashes in xdcr and orphan replication
  to be displayed in the UI we need to delete such
  database
- this code is executed from xdc_rep_manager:init
  since xdc_rep_manager preceeds xdc_rdoc_replication_srv
  in the supervisor spec
- xdc_rdoc_replication_srv now assumes that correct _replicator
  database is created by xdc_rep_manager and will
  not try to create it itself

Change-Id: I7dd8c7b7fa70c0a74a0bdc8d8c2128595ef7e0be
Reviewed-on: http://review.couchbase.org/28423
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>