MB-11861 Remove incorrect assertion from the flusher.

As VBucket memory deletion can be interleaved with the flusher,
the assertion check on the high sequence number is not required.

Change-Id: Iac80a68f47ec315d01cb18133c0282396e6268cb
Reviewed-on: http://review.couchbase.org/40151
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>