Fix memory leak in vbucket deletion

If there are any bg fetch items then we must remember to explicitly
delete the value from them otherwise we will leak memory.

Change-Id: Id7f7ac176265a41ae7baf9e084a92b26ad068bd5
Reviewed-on: http://review.couchbase.org/41571
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>