MB-7656 Remove not used anymore btree code

Not used anywhere.

Change-Id: Ie9451d79a2925a68b757d2177faadbc322b5e694
Reviewed-on: http://review.couchbase.org/28742
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>