ol.structs.RBush#remove() is unreliable

This was discovered when investigating issues with #1283. The issue can be reproduced with this [modified test case](https://github.com/ahocevar/ol3/compare/rbush-fail). The same operation does not fail in the [original RBush implemenatation](https://github.com/ahocevar/rbush/compare/pass-remove).