Refactoring of RemovableDriveManager:

1) On Windows and Linux, the device enumeration now runs at a background
   thread, while it ran on the UI thread on idle, which may have been
   blocking on some rare Windows setups, see GH #3515 #3733 #3746 #3766
2) On OSX, the device enumeration now relies on OS callback, no
   polling is required.
3) Refactored for cleaner interface.

src/slic3r/GUI/Plater.hpp

@@ -213,7 +213,6 @@ public:
     void fix_through_netfabb(const int obj_idx, const int vol_idx = -1);
     void send_gcode();
 	void eject_drive();
-	void drive_ejected_callback();
 
     void take_snapshot(const std::string &snapshot_name);
     void take_snapshot(const wxString &snapshot_name);