Remove an Into<Option<T>>

Technically a bugfix because this made it sort of unusable; instead,
users need to explicitly `Some(Box::new(receiver))`.

src/voice/handler.rs

@@ -228,8 +228,8 @@ impl Handler {
     /// can pass in just a boxed receiver, and do not need to specify `Some`.
     ///
     /// Pass `None` to drop the current receiver, if one exists.
-    pub fn listen<O: Into<Option<Box<AudioReceiver>>>>(&mut self, receiver: O) {
-        self.send(VoiceStatus::SetReceiver(receiver.into()))
+    pub fn listen(&mut self, receiver: Option<Box<AudioReceiver>>) {
+        self.send(VoiceStatus::SetReceiver(receiver))
     }
 
     /// Sets whether the current connection is to be muted.