Fix warning: remove unused variable and clean up imports

Remove the unused config variable clone from the listener accept loop where
it was captured by the thread closure but never referenced. Remove the
unused PrivateKeyDer import from rustls pki_types. Prefix the catch-all
match variable with underscore to suppress the unused variable warning in
the dispatch function. Keep the required parentheses around impl Trait
syntax in argument position as required by Rust 2024 edition for
disambiguation. Add MIT LICENSE file.