Platform: clean up leftovers after old deprecated viewportEvent().

There used to be a backwards compatibility code proxying to a function
that took just a single const Vector2& and this was left after it got
removed.

src/Magnum/Platform/AbstractXApplication.cpp

@@ -191,10 +191,7 @@ bool AbstractXApplication::mainLoopIteration() {
     return !(_flags & Flag::Exit);
 }
 
-void AbstractXApplication::viewportEvent(ViewportEvent& event) {
-    static_cast<void>(event);
-}
-
+void AbstractXApplication::viewportEvent(ViewportEvent&) {}
 void AbstractXApplication::keyPressEvent(KeyEvent&) {}
 void AbstractXApplication::keyReleaseEvent(KeyEvent&) {}
 void AbstractXApplication::mousePressEvent(MouseEvent&) {}

src/Magnum/Platform/AndroidApplication.cpp

@@ -186,10 +186,7 @@ void AndroidApplication::redraw() {
     _flags |= Flag::Redraw;
 }
 
-void AndroidApplication::viewportEvent(ViewportEvent& event) {
-    static_cast<void>(event);
-}
-
+void AndroidApplication::viewportEvent(ViewportEvent&) {}
 void AndroidApplication::mousePressEvent(MouseEvent&) {}
 void AndroidApplication::mouseReleaseEvent(MouseEvent&) {}
 void AndroidApplication::mouseMoveEvent(MouseMoveEvent&) {}

src/Magnum/Platform/GlfwApplication.cpp

@@ -866,10 +866,7 @@ void GlfwApplication::exitEvent(ExitEvent& event) {
     event.setAccepted();
 }
 
-void GlfwApplication::viewportEvent(ViewportEvent& event) {
-    static_cast<void>(event);
-}
-
+void GlfwApplication::viewportEvent(ViewportEvent& event) {}
 void GlfwApplication::keyPressEvent(KeyEvent&) {}
 void GlfwApplication::keyReleaseEvent(KeyEvent&) {}
 void GlfwApplication::mousePressEvent(MouseEvent&) {}

src/Magnum/Platform/Sdl2Application.cpp

@@ -1171,10 +1171,7 @@ void Sdl2Application::anyEvent(SDL_Event&) {
     _flags |= Flag::NoAnyEvent;
 }
 
-void Sdl2Application::viewportEvent(ViewportEvent& event) {
-    static_cast<void>(event);
-}
-
+void Sdl2Application::viewportEvent(ViewportEvent&) {}
 void Sdl2Application::keyPressEvent(KeyEvent&) {}
 void Sdl2Application::keyReleaseEvent(KeyEvent&) {}
 void Sdl2Application::mousePressEvent(MouseEvent&) {}