RPG: Fix some editor issues

[Hypexion]

A few issues have seeped into the editor during some changes. This problems should now be fixed:

• Some stair creation code was still working as if the staircase would be lined with walls.
• Removing a staircase was still removing adjoining walls.
• Removing arrows would not remove orb associations when appropriate.

Additionally, a removed staircase will now be replaced with the most recently select floor type.