Remove reference-transaction hook

There's a bug in the reference-transaction implementation in git-core which will cause interleaving calls to the reference-transaction hook to execute another hook by accident. While a fix has been proposed upstream, the earliest it may be available is with Git v2.28.1. So let's remove our reference-transaction hook for now to not expose this broken behaviour.

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I'm disabling this hook in Gitaly so that we can move ahead with the upgrade to Git v2.28.0. Reverting all the different MRs towards that version would be a major pain.

Fixes gitlab#233964 (closed)