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/termios.cpp

@@ -51,10 +51,8 @@ int tcsetattr(int fd, int optional_actions, const struct termios* t)
     return -1;
 }
 
-int tcflow(int fd, int action)
+int tcflow([[maybe_unused]] int fd, [[maybe_unused]] int action)
 {
-    (void)fd;
-    (void)action;
     ASSERT_NOT_REACHED();
 }