Remove the dispatcher

All tasks have been migrated over to the global scheduler so
we no longer need the dispatcher.

Change-Id: Ie9e0973e66c0d998d7f603ca169c14d2b0c447da
Reviewed-on: http://review.couchbase.org/30898
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>