don't check if num_gateways is zero

condition is redundant according to "def num_gateways" implementation.

Change-Id: I6cc7059c311cc7e0002ff1ecbf9b008f6b24c010
Reviewed-on: http://review.couchbase.org/37467
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Li Yang <lichenyang2@gmail.com>