MB-41321: 3/4 removeDeletedDoc redundant collection item count adjustment

During rollback's removeDeletedDoc function we read lock the
manifest and decrement the collection's item count, yet at
the end of rollback we throw away the manifest and create
a new one using the rollback point, making the decrement (and
read-locking) redundant.

Change-Id: I1803d516c66841fd92ccefc843a62db9e17b2fe4
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/137408
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>

Author: jimwwalker

engines/ep/src/ep_bucket.cc

@@ -1564,9 +1564,6 @@ class EPDiskRollbackCB : public RollbackCB {
             // Irrespective of if the in-memory delete succeeded; the document
             // doesn't exist on disk; so decrement the item count.
             vb.decrNumTotalItems();
-            // @todo: investigate if this is needed. Once rollback is done we
-            // reset all collections stats from the rollback point
-            vb.getManifest().lock(item.getKey()).decrementItemCount();
         }
     }