MB-8890: Remove assertion for deleted/non-existent tmp item

This assertion is invalid since it is possible that a background
fetch for an item is updated through a front end operation and
then evicted from memory before the background fetch completes.

Change-Id: I3d7638c10bd88721f3a30c7aa4e143fa476b0a14
Reviewed-on: http://review.couchbase.org/28403
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>