Similar to "Cancel all tasks" UI, change /botlist to show a button to "Delete all dead bots". Preferably requiring at least one filter to prevent misclicks.

This would result in the equivalent of a bash for loop calling "swarming.py bot-delete". I don't think it's worth having an API, doing it as a loop in javascript is likely fine for the number of times this is expected to be used and the number of bots to be deleted, generally a few tens, can accept degraded behavior in the thousands, as this is rarely time critical.