Remove Costmap 2D

[amock]

Removed costmap 2d as discussed in #346

[Rayman]

For me its a bit strange that the costmap 2d is not included in the core mbf repo. This means mbf is not a plug and play replacement for navigation2. This means adoption will be slower.

You can also choose to just not release these packages yet, and wait until they are migrated?

(I'm not a maintainer, I've only used mbf during ROS 1)

[amock]

Actually, I don't think MBF itself can be a plug-and-play replacement for Nav2; only a 2d costmap-based implementation of MBF can be. If you want, you can still create a separate repository with such an implementation, which could serve as a plug-and-play replacement for Nav2.

I believe this step simply makes MBF truly and more obviously independent of any specific map representation. That said, I'm not sure if this is the best approach, which is why I started the discussion in issue #346.