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/LibChess/Chess.cpp

@@ -770,8 +770,7 @@ Board::Result Board::game_result() const
         return Result::InsufficientMaterial;
 
     bool are_legal_moves = false;
-    generate_moves([&](Move m) {
-        (void)m;
+    generate_moves([&]([[maybe_unused]] Move m) {
         are_legal_moves = true;
         return IterationDecision::Break;
     });