Mulebot lag fix

Fix for the server crippling MULEbot lag.
- Removes a spawn() which is no longer needed since bots have their own place in the master controller which run them independently.
- Fixes an infinite loop that results in extreme server lag.
- MULEs will no longer calculate a path to a destination not matching their own when receiving a signal through the radio controller.

Note: This could potentially make MULEbots less effective at handling pathing failures, but it is a lot better than being able to completely shut down the server.

Infinite loop details:
<i>When a MULEbot is commanded to proceed to a turf in which it is already located, and is not set to automatically return home, it call its at_target() proc infinitely. That proc sends a signal to all the other MULEbots, Cargo PDAs, and beacons. When other MULEs receive this signal, they will pointlessly calculate a path to the sending MULE, regardless of their own status. If the MULE got stuck in such a loop, it would broadcast its status several times a second, causing distant MULEs to calculate a path to the source MULE, creating A-Star lag which exponentially increases as more MULEs are added.</i>

Please comment your methods of replicating any further bugs, and I will work to
patch it.