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/Random.h

@@ -43,16 +43,13 @@
 
 namespace AK {
 
-inline void fill_with_random(void* buffer, size_t length)
+inline void fill_with_random([[maybe_unused]] void* buffer, [[maybe_unused]] size_t length)
 {
 #if defined(__serenity__)
     arc4random_buf(buffer, length);
 #elif defined(OSS_FUZZ)
-    (void)buffer;
-    (void)length;
 #elif defined(__unix__) or defined(__APPLE__)
-    int rc = getentropy(buffer, length);
-    (void)rc;
+    [[maybe_unused]] int rc = getentropy(buffer, length);
 #endif
 }