Remove some duplicate code, Qt does the same already

Recently Qt implemented something similar. Since we only support
aero snap with Qt 5.15 we dont need KDDW's native win32 code anymore

cc issue #121

src/private/DragController.cpp

@@ -256,10 +256,6 @@ void StateDragging::onEntry()
 
             QWindow *window = fw->windowHandle();
             window->startSystemMove();
-
-            // Mouse press was done in another window, so we need to ungrab
-            ReleaseCapture();
-            PostMessage(hwnd, WM_SYSCOMMAND, 0xF012, 0); // SC_DRAGMOVE
         }
 # endif
 #endif