Keeping worse bots

There are a few bots doing worse than sample.SittingDuck, but imo we should keep them, at least not removing them in one step.

1. Removing a lot of bots in roborumble in a short period of time disables the comparation between different versions of every bot in APS, survival, PWIN, etc. As they all depends highly on the distribution of the participants. Doing so makes the recorded APS, Survival and PWIN in version history completely useless, and you can't even reload the scores as the the score against those removed bots are always counted in inactive versions.

2. They are a great indicator of whether your bot has some serious bug that happens rarely. And we DO need enough weak bots to have the chance to trigger it.

3. They don't waste time in roborumble as they die too fast, making a battle almost done immediately.

Anyway, I think we should not remove any bot except your own without discussion, except when those bots are violating rules.