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.

AK/StdLibExtras.h

@@ -26,8 +26,6 @@
 
 #pragma once
 
-#define UNUSED_PARAM(x) (void)x
-
 constexpr unsigned round_up_to_power_of_two(unsigned value, unsigned power_of_two)
 {
     return ((value - 1) & ~(power_of_two - 1)) + power_of_two;