[BP] 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: I2220b2f735024da4469664c0c0ab9e8145aa0b1e
Reviewed-on: http://review.couchbase.org/43303
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>