drop config key for picking old xdcr path

Change-Id: I48e55172bd482d08642e69f219ea912cb0f78144
Reviewed-on: http://review.couchbase.org/38460
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>