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.

Kernel/Syscalls/alarm.cpp

@@ -49,7 +49,7 @@ unsigned Process::sys$alarm(unsigned seconds)
         auto deadline = TimeManagement::the().current_time(CLOCK_REALTIME).value();
         timespec_add(deadline, { seconds, 0 }, deadline);
         m_alarm_timer = TimerQueue::the().add_timer_without_id(CLOCK_REALTIME, deadline, [this]() {
-            (void)send_signal(SIGALRM, nullptr);
+            [[maybe_unused]] auto rc = send_signal(SIGALRM, nullptr);
         });
     }
     return previous_alarm_remaining;