Remove state_changed_on for metrics gap collection

[agrare]

The need for state_changed_on being set for metrics collection purposes is causing performance issues for refresh because the custom raw_power_state= method must be invoked in ruby code and cannot be processed by batch saving.

If there were another way to detect gaps in metrics data or remove the detection entirely then the state_changed_on column would no longer be needed.

[agrare]

cc @Fryguy

[miq-bot]

This issue has been automatically marked as stale because it has not been updated for at least 6 months.

If you can still reproduce this issue on the current release or on master, please reply with all of the information you have about it in order to keep the issue open.

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[Fryguy]

We should look into why this was added in the first place. I think it had something to do with synchronizing with the EmsRefreshCoreWorker.