compose: Remove obsolete references to replying_to_message.

The `replying_to_message` field was used in some
early versions of compose fade, but it has no more
use in the current code.

The drafts implementation didn't really make any sense,
anyway, as we were claiming to reply to the same
message we were drafting.

static/js/compose_actions.js

@@ -328,7 +328,6 @@ exports.respond_to_message = function (opts) {
     }
     exports.start(msg_type, {stream: stream, subject: topic,
                              private_message_recipient: pm_recipient,
-                             replying_to_message: message,
                              trigger: opts.trigger});
 
 };