Remove skip metric hacks, remove engine-gecko

[badboy]

Fixes #697

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I think we can do this now?
A dry-run locally succeeds.

python3 -m probe_scraper.runner --dry-run --cache-dir tmp/cache --out-dir tmp/out --glean --glean-repo firefox-klar-android --glean-repo service-nimbus --glean-repo nimbus --glean-limit-date 2024-04-01 --update --output-bucket=gs://probe-scraper-prod-artifacts/ --glean-repo firefox-desktop --glean-repo gecko --glean-repo engine-gecko --glean-repo firefox-android-release

might want some more testing before we actually land this.

[badboy]

hopefully not needed anymore if I can land https://bugzilla.mozilla.org/show_bug.cgi?id=1943037