MB-12305: Don't update the snap start/end seqno in tap mutation

We do this in the queueDirty function so this code is not needed. It
also appeared to cause a race in updating the snapshot start and end
sequence numbers so removing it should solve the bug linked above.

Change-Id: Ia8fa36df958be9147ea208ba9ebd78496048ebb4
Reviewed-on: http://review.couchbase.org/42503
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>