util: Kill off rtrim() helper.

I am 99% sure we can rely on trimRight() and
trim() being available in all browsers that
we support.  I verified in FF.

This removes the util dependency from both
modules touched here.

static/js/compose_state.js

@@ -1,4 +1,3 @@
-const util = require("./util");
 let message_type = false; // 'stream', 'private', or false-y
 
 exports.set_message_type = function (msg_type) {
@@ -32,7 +31,7 @@ function get_or_set(fieldname, keep_leading_whitespace) {
         if (newval !== undefined) {
             elem.val(newval);
         }
-        return keep_leading_whitespace ? util.rtrim(oldval) : $.trim(oldval);
+        return keep_leading_whitespace ? oldval.trimRight() : oldval.trim();
     };
 }