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: {
-                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;
-            }
+            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;
 
             case SDL_MOUSEMOTION: {
                 MouseMoveEvent e({event.motion.x, event.motion.y}, {event.motion.xrel, event.motion.yrel}, static_cast<MouseMoveEvent::Button>(event.motion.state));