Platform: revert "Allow horizontal-only scrolling"

Will be replaced by proper scroll event. This reverts commit
c817f8412e.

src/Magnum/Platform/Sdl2Application.cpp

@@ -384,15 +384,15 @@ void Sdl2Application::mainLoop() {
                 event.type == SDL_MOUSEBUTTONDOWN ? mousePressEvent(e) : mouseReleaseEvent(e);
             } break;
 
-            case SDL_MOUSEWHEEL: {
+            case SDL_MOUSEWHEEL:
+                if(event.wheel.y != 0) {
                     MouseEvent e(event.wheel.y > 0 ? MouseEvent::Button::WheelUp : MouseEvent::Button::WheelDown, {event.wheel.x, event.wheel.y}
                         #ifndef CORRADE_TARGET_EMSCRIPTEN
                         , 0
                         #endif
                         );
                     mousePressEvent(e);
-                break;
+                } break;
-            }
 
             case SDL_MOUSEMOTION: {
                 MouseMoveEvent e({event.motion.x, event.motion.y}, {event.motion.xrel, event.motion.yrel}, static_cast<MouseMoveEvent::Button>(event.motion.state));