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/LibLine/Editor.cpp

@@ -1480,7 +1480,7 @@ Vector<size_t, 2> Editor::vt_dsr()
 
     do {
         more_junk_to_read = false;
-        (void)select(1, &readfds, nullptr, nullptr, &timeout);
+        [[maybe_unused]] auto rc = select(1, &readfds, nullptr, nullptr, &timeout);
         if (FD_ISSET(0, &readfds)) {
             auto nread = read(0, buf, 16);
             if (nread < 0) {