Remove minimetrics (#94274)

The Sentry metrics product never landed so we can remove this from the
code base.

This is the second attempt after this:
#93595

Now the other references to `CompositeExperimentalMetricsBackend` in
`ops` and `getsentry` repo have been removed, this can be merged.

Also did an org wide code search on Github and the only references are
from archived repos and one very old demo project:
https://github.com/search?q=org%3Agetsentry%20CompositeExperimentalMetricsBackend&type=code

Author: antonpirker