Clean the final codebase

To make our final codebase the cleanest possible, before setting it as finished, we must:

[ ] remove the functions/methods that aren't called anywhere.
[ ] put every method that can be in the private section or the protected section.
[ ] remove every #include unused in the file where the include is.
[ ] put const qualifiers everywhere where it can be and we forgot to do it before.
[ ] check every doxygen comment to make sure all are present and well formatted.

Saphirac / ft_irc