use the mgo built in remove rather than a custom one
added comment regarding the use of mgo.Remove to cleanup metrics
Merge pull request #681 from mattyw/delete-metric-batch-bulk

use the mgo built in remove rather than a custom one

Previously we had our own function for deleting a metric batch during the Cleanup call. All we need to do is delete metrics based on a certain selection so we can just use collection.Remove() from mgo.