MB-37637: Remove mem_used check from mem_stats test
MB-37637: Remove mem_used check from mem_stats test

This test is currently failing sporadically as it expects mem_used
to have increased or remained the same after BG fetching an item,
when compared to the mem_used before it evicted that item, in reality
it sometimes decreases. mem_used can move for a large variety of
reasons, for example a background task that creates a visitor which
is allocated on the heap, so it is very difficult to assert what
happens to mem_used. Given we check value_size which SHOULD be correct
after the BG fetch also, just remove the mem_used check.

(cherry picked from commit 899273a32330af8077b1e42b91127a9a5c46aa04)

Change-Id: I5a126652c56f178aa26c04411c5561d66a27c8a2
Reviewed-on: http://review.couchbase.org/122173
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Ben Huddleston <6c0d3f0e87b08a8c49786ca528f2c4b1ad3d36f3@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine