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/LibPthread/pthread_once.cpp

@@ -37,7 +37,7 @@ enum State : i32 {
     PERFORMING_WITH_WAITERS,
 };
 
-int pthread_once(pthread_once_t* self, void (*callback)(void))
+int pthread_once(pthread_once_t* self, void (*callback)())
 {
     auto& state = reinterpret_cast<Atomic<State>&>(*self);