Everywhere: Switch from (void) to [[maybe_unused]] (#4473)

Problem:
- `(void)` simply casts the expression to void. This is understood to
  indicate that it is ignored, but this is really a compiler trick to
  get the compiler to not generate a warning.

Solution:
- Use the `[[maybe_unused]]` attribute to indicate the value is unused.

Note:
- Functions taking a `(void)` argument list have also been changed to
  `()` because this is not needed and shows up in the same grep
  command.

Libraries/LibWeb/Page/EventHandler.cpp

@@ -301,9 +301,8 @@ bool EventHandler::handle_mousemove(const Gfx::IntPoint& position, unsigned butt
     return true;
 }
 
-void EventHandler::dump_selection(const char* event_name) const
+void EventHandler::dump_selection([[maybe_unused]] const char* event_name) const
 {
-    UNUSED_PARAM(event_name);
 #ifdef SELECTION_DEBUG
     dbg() << event_name << " selection start: "
           << layout_root()->selection().start().layout_node << ":" << layout_root()->selection().start().index_in_node << ", end: "