Create binding for unmountComponentAtNode in bridgeless mode

Summary: Right now we register ReactFabric as a callable module with the bridge so that we can call `ReactFabric.unmountComponentAtNode` in `ReactInstanceManager.detachViewFromInstance`. In bridgeless mode we don't have callable modules, so I'm just setting a global variable that can be called from C++ instead. Using this in a new `unmount` method in FabricUIManager.

Reviewed By: shergin, mdvacca

Differential Revision: D16273720

fbshipit-source-id: 95edb16da6566113a58babda3ebdf0fc4e39f8b0