Fixes T4202. We have old code in MetaMTA which implements gradual backoff and maximum retries.

However, we have more general code in the task queue which does this, too. We can just use the more general stuff in the task queue; it obsoletes the specific stuff in MetaMTA, which is more complex and ran into some kind of issue in T4202.

Remove retryCount, nextRetry (obsoleted by task queue retry mechanisms) and "simulated failures" (no longer in use).

Generally, modern infrastructure has replaced these mechanisms with more general ones.

• Sent mail.
• Observed unsendable mail failing in reasonable ways in the queue.