MB-9096: removed unused start_rebalance_old_style function

Change-Id: Icc798c463d839237c2bcea1c805125f85dd23009
Reviewed-on: http://review.couchbase.org/29321
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>