Update test_flush_stats to remove mem_used checks

When accurate memory tracking is enabled, there is too many
allocations going on 'under the hood' to be able to check that
mem_used increases relative to a baseline when only two small keys are
added. Therefore just remove the mem_used checks from this testcase.

Change-Id: I841b8601234a88f1b0b2417b51178ede9091e706
Reviewed-on: http://review.couchbase.org/42240
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>