Remove unused thread member: last_checked
Remove unused thread member: last_checked

Change-Id: Ica79a9c5039b59739f18d06587f832bdff4209c8
Reviewed-on: http://review.couchbase.org/79267
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

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