Allow unqueuing of queued messages in MessageQueue

[SkyBirdSoar]

Prevent possible spam to the channel even with throttling
Can happen if:

• Bot can evaluate strings of code with a command and the strings of code contain a loop which spams m.reply.

[dominikh]

If you can add logic to unqueue messages you can just as well add logic to not send the message in the first place, can't you?

And allowing the bot to execute arbitrary user-provided Ruby code is a security nightmare waiting to happen.