compose: Remove unnecessary snapshot_message call.

The current logic that we have is as follows:

* If a message is locally echoed, the draft is stored via the locally
rendered message, and that system takes care of it.  So no need to
store it here.

* If the message isn't locally echoed, we don't close the compose box
until, so the content is safe here as well.  It'll be saved as a draft
if the compose box is later closed due to a failure sending.

static/js/compose.js

@@ -565,7 +565,6 @@ function send_message(request) {
     if (request === undefined) {
         request = create_message_object();
     }
-    exports.snapshot_message(request);
 
     if (request.type === "private") {
         request.to = JSON.stringify(request.to);