widgets: Remove spammy blueslip.info for submessages.

Having submessages will become common enough that the
info message here is too spammy, and there are other
ways to observe incoming submessages if you're doing dev
debugging, which this was originally written for.

static/js/submessage.js

@@ -43,8 +43,6 @@ exports.process_submessages = function (in_opts) {
         return;
     }
 
-    blueslip.info('submessages found for message id: ' + message_id);
-
     var row = in_opts.row;
 
     // Right now, our only use of submessages is widgets.