Remove redundant imports

These became warnings in a recent nightly.

The TryFrom/TryInto imports were missed in
4089891f6c, but the rest are truly
redundant.

tokio-xmpp/examples/echo_bot.rs

@@ -2,7 +2,6 @@ use futures::stream::StreamExt;
 use std::env::args;
 use std::process::exit;
 use std::str::FromStr;
-use tokio;
 use tokio_xmpp::AsyncClient as Client;
 use xmpp_parsers::message::{Body, Message, MessageType};
 use xmpp_parsers::presence::{Presence, Show as PresenceShow, Type as PresenceType};