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/LibHTTP/Job.cpp

@@ -271,11 +271,10 @@ void Job::on_socket_connected()
 
                     // we've read everything, now let's get the next chunk
                     size = -1;
-                    auto line = read_line(PAGE_SIZE);
+                    [[maybe_unused]] auto line = read_line(PAGE_SIZE);
 #ifdef JOB_DEBUG
                     dbg() << "Line following (should be empty): _" << line << "_";
 #endif
-                    (void)line;
                 }
                 m_current_chunk_remaining_size = size;
             }