Remove system_usage_data_monthly from usage_data.rb

What does this MR do and why?

Related to #421506 (closed)

We want to move some metrics from usage_data.rb into metric definitions.

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Before	After

How to set up and validate locally

To test this, we need to make sure that the metrics work the same as before.

1. While on master branch, open the rails console and generate a service ping: sp = Gitlab::Usage::ServicePingReport.for(output: :all_metrics_values)
2. Check out this MR's branch and run reload! in the rails console
3. Generate a new service ping: sp2 = Gitlab::Usage::ServicePingReport.for(output: :all_metrics_values)
4. We should have the same metric values for the updated metrics: sp['counts_monthly'] == sp2['counts_monthly'] should return true.

MR acceptance checklist

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