Driver: Fix incorrect leave behaviour in Drop handler

Sending poison messages should suffice to kill the voice session: attempting to `.leave()`. Fixes #88.

This was tested using `cargo make ready` and the modified `serenity/voice/` example.

src/driver/mod.rs

@@ -308,7 +308,6 @@ impl Drop for Driver {
     /// Leaves the current connected voice channel, if connected to one, and
     /// forgets all configurations relevant to this Handler.
     fn drop(&mut self) {
-        self.leave();
         let _ = self.sender.send(CoreMessage::Poison);
     }
 }