remove `retry_on_integrity_error` wrapper for persist_events (#7848)

As far as I can tell from the sentry logs, the only time this has actually done
anything in the last two years is when we had two master workers running at
once, and even then, it made a bit of a mess of it (see
https://github.com/matrix-org/synapse/issues/7845#issuecomment-658238739).

Generally I feel like this code is doing more harm than good.