Remove contents of `metrics.yaml` as a defense in depth against Glean metrics reporting
The Fenix metrics that are potentially collected are defined in
metrcis.yaml. We likely can just remove its content (maybe we need to
adjust `GleanMetricsService.kt) to be sure nothing is collected and sent
Another option would be to point to an own, empty .yaml file with
This could be a good workaround for tor-browser-build#40085 (closed), until a real fix lands and bubbles up to all of our components.