No need to pass the vbucket object to vb_delete_task

Change-Id: I0ca93f00f5d11fac435e63b6ad7f95549f170c17
Reviewed-on: http://review.couchbase.org/39862
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>