Fix ol.structs.RBush#remove

This PR fixes a problem found by @fredj. `ol.structs.RBush#remove` would occasionally fail when removing values from large R-Bushes.

Rather than trying to find the subtle problem in the iterative `remove_` code that maintains its own stack, I implemented a simpler recursive solution that uses the call stack. Hopefully this one is easier to reason about, as well as being correct.