MB-16686: Remove sanity check while adding TAP over DCP

MB-16686: Remove sanity check while adding TAP over DCP

This check isn't accurate as certain TAP messages from
the producer carry no vbucket information - initialized to
zero (expected), as they aren't vbucket specific operations.
In such a scenario, if the TAP consumer needs to be created,
it wouldn't be allowed to if a DCP passive stream exists
for vbucket 0. This would break an online upgrade.

Change-Id: I310b9cf4dbaf652c233cba02de7ca72469efa89d
Reviewed-on: http://review.couchbase.org/56564
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>