SYNC-5090 remove unneeded metrics

[bendk]

We've been experimenting with new metrics dashboards, but when these were plotted they didn't seem useful at all.

https://mozilla-hub.atlassian.net/browse/SYNC-5090

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[bendk]

I guess we'll need to update probe-scraper since this PR deletes the fxa-client metrics.yaml file.

mozilla/probe-scraper#986

[badboy]

as mentioned over in mozilla/probe-scraper#990 this did end up causing an issue in our pipeline.
We had to revert.
The easiest way to unblock this is to keep the file here empty, so just the basics:

---
$schema: moz://mozilla.org/schemas/glean/metrics/2-0-0