Remove CallbackRegistry

I made this change because I think CallbackRegistry is an unnecessary level of indirection. I am not super happy that I added `getInstanceHandle` to the injection of EventPluginHub and I'd understand if this is not something we want - so consider this change an RFC. I had to move the injection code to avoid a circular dependency.

Personally I think it makes React a bit easier to follow (follow as in "where is this function defined") and if we'd need to switch out the put/get/deleteListener functions with a different type of registry it should be easy to revert part of this diff.