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/LibC/sched.cpp

@@ -36,15 +36,13 @@ int sched_yield()
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
 
-int sched_get_priority_min(int policy)
+int sched_get_priority_min([[maybe_unused]] int policy)
 {
-    (void)policy;
     return 0; // Idle
 }
 
-int sched_get_priority_max(int policy)
+int sched_get_priority_max([[maybe_unused]] int policy)
 {
-    (void)policy;
     return 3; // High
 }