notifications for failed tasks on geckoview bump aren't working

[jcristau]

ac_taskgraph has code to send a notification to AC and geckoview developers when tasks fail on geckoview-bump PRs, however as far as I can tell this hasn't worked for a couple of years (e.g. it expects branch names that haven't been used since https://github.com/mozilla-mobile/relbot/pull/43). Would it be OK to rip that out or should we try to fix/revive it? (And if the latter, maybe only in the firefox-android repo?)

cc @JohanLorenzo @rvandermeulen

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