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.

Applications/IRCClient/IRCClient.cpp

@@ -773,9 +773,8 @@ void IRCClient::handle_rpl_whoisuser(const Message& msg)
     auto& nick = msg.arguments[1];
     auto& username = msg.arguments[2];
     auto& host = msg.arguments[3];
-    auto& asterisk = msg.arguments[4];
+    [[maybe_unused]] auto& asterisk = msg.arguments[4];
     auto& realname = msg.arguments[5];
-    (void)asterisk;
     add_server_message(String::formatted("* {} is {}@{}, real name: {}", nick, username, host, realname));
 }