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/Thread.cpp

@@ -213,7 +213,7 @@ void Thread::die_if_needed()
         return;
 
     u32 unlock_count;
-    (void)unlock_process_if_locked(unlock_count);
+    [[maybe_unused]] auto rc = unlock_process_if_locked(unlock_count);
 
     ScopedCritical critical;
     set_should_die();
@@ -240,7 +240,7 @@ void Thread::exit(void* exit_value)
     m_join_condition.thread_did_exit(exit_value);
     set_should_die();
     u32 unlock_count;
-    (void)unlock_process_if_locked(unlock_count);
+    [[maybe_unused]] auto rc = unlock_process_if_locked(unlock_count);
     die_if_needed();
 }