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.cpp

@@ -460,19 +460,13 @@ int pthread_attr_setstacksize(pthread_attr_t* attributes, size_t stack_size)
     return 0;
 }
 
-int pthread_getschedparam(pthread_t thread, int* policy, struct sched_param* param)
+int pthread_getschedparam([[maybe_unused]] pthread_t thread, [[maybe_unused]] int* policy, [[maybe_unused]] struct sched_param* param)
 {
-    (void)thread;
-    (void)policy;
-    (void)param;
     return 0;
 }
 
-int pthread_setschedparam(pthread_t thread, int policy, const struct sched_param* param)
+int pthread_setschedparam([[maybe_unused]] pthread_t thread, [[maybe_unused]] int policy, [[maybe_unused]] const struct sched_param* param)
 {
-    (void)thread;
-    (void)policy;
-    (void)param;
     return 0;
 }