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/execve.cpp

@@ -371,7 +371,7 @@ int Process::do_exec(NonnullRefPtr<FileDescription> main_program_description, Ve
         new_main_thread->set_state(Thread::State::Runnable);
     }
     u32 lock_count_to_restore;
-    (void)big_lock().force_unlock_if_locked(lock_count_to_restore);
+    [[maybe_unused]] auto rc = big_lock().force_unlock_if_locked(lock_count_to_restore);
     ASSERT_INTERRUPTS_DISABLED();
     ASSERT(Processor::current().in_critical());
     return 0;