Bug 1463085 - Remove unnecessary self-assignment. r=maja_zf

Prior to bug 1400256 we used to have to deserialise the web
element received from chrome space manually, and presumably this
self-assignment is a leftover from that time.

This fixes an eslint error upon upgrading to eslint version 5.

Thanks-to: Mark Banner <standard8@mozilla.com>
MozReview-Commit-ID: A46wK71AwBe

--HG--
extra : rebase_source : 2ad04f87b31cb369c7c3acf6a50a3bcbd493a735

testing/marionette/listener.js

@@ -1150,10 +1150,6 @@ async function findElementContent(strategy, selector, opts = {}) {
   }
 
   opts.all = false;
-  if (opts.startNode) {
-    opts.startNode = opts.startNode;
-  }
-
   let el = await element.find(curContainer, strategy, selector, opts);
   return seenEls.add(el);
 }