Remove date_created from MetricRow

I've noticed that pipelines fail on some metrics during metric data load. Turns out that we stopped using timestamps in definition filenames and the data loader fails to handle that.

This fixes the data loader raising an exception on metric filenames not containing a timestamp by removing date_created and should subsequently get the pipelines green again.

We don't do anything with date_created from the filename anyway, so it's a redundant attribute.