Fix server handler and listener warnings: remove unnecessary parentheses and unused imports

Clean up compilation warnings in the pwr-server crate. In handler.rs, the
send_server_msg function signature used unnecessary parentheses around
impl Write which triggered the unused_parens lint. In listener.rs, the
PrivateKeyDer import was unused since the key is loaded via
rustls_pemfile::private_key which returns a generic PrivateKeyDer type
that does not need to be explicitly imported.