Remove unused robots in dev mode too

[Adimote]

I'm not entirely sure what the side-effects are of reordering these events, advice would be more than welcome!

But the benefits of this is my simulator runs at 5x speed with 1 robot in the arena, and 3x speed with 4 robots in the arena, so it's about a 66% simulation speed improvement

[PeterJCLaw]

Immediate thoughts, but haven't thought much yet:

• we should test this in each of the cases of
  • the default robot code location
  • just zone-0
  • just some other zone
  • what happens if there is no user-code found
• does this make it less obvious to competitors that there will be other robots in the arena?
• does this make it less obvious to competitors that they can test their code in other starting zones?

[Adimote]

• the default robot code location

Robot was deleted :( will have to fix before submitting.

• just zone-0

This works

• just some other zone

This also works

• what happens if there is no user-code found

Seems to behave correctly, a robot.py was created!

• does this make it less obvious to competitors that there will be other robots in the arena?

I don't think so? Maybe we can mitigate this by adding a starting box under where the robots start

• does this make it less obvious to competitors that they can test their code in other starting zones?

I think we should fix this by documenting it better