Remove es6-shim dependency

[Kukkimonsuta]

As far as I can tell it's not being used and if I've missed something and is being imported somewhere, library still shouldn't modify global environment.

[elboman]

I think we added it with previous version of typescript for compiling the location strategies (which uses Promises) before we moved them to the core. Also typescript 2.x should take care of them now with the lib option, should be safe to remove it.