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/Scheduler.cpp

@@ -253,14 +253,13 @@ bool Scheduler::yield()
     return true;
 }
 
-bool Scheduler::donate_to_and_switch(Thread* beneficiary, const char* reason)
+bool Scheduler::donate_to_and_switch(Thread* beneficiary, [[maybe_unused]] const char* reason)
 {
     ASSERT(g_scheduler_lock.own_lock());
 
     auto& proc = Processor::current();
     ASSERT(proc.in_critical() == 1);
 
-    (void)reason;
     unsigned ticks_left = Thread::current()->ticks_left();
     if (!beneficiary || beneficiary->state() != Thread::Runnable || ticks_left <= 1)
         return Scheduler::yield();