Remove comment thread prerendering

This should not be necessary anymore since all comments are
"pre-rendered" in the gr-diff-host. I compared before and after with the
Chrome profiler for the file [1] of the original report [2], and could
not find a difference.

[1] /c/chromiumos/third_party/coreboot/+/257398/25/src/soc/mediatek/mt8173/dramc_pi_calibration_api.c
[2] https://bugs.chromium.org/p/gerrit/issues/detail?id=4335

Change-Id: If030867ddf0d7e8aba1252c06242965eca5c08c0

polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.html

@@ -156,10 +156,6 @@ limitations under the License.
           }
 
           this._layers = layers;
-
-          this.async(() => {
-            this._preRenderThread();
-          });
         },
 
         render(comments, prefs) {
@@ -448,25 +444,6 @@ limitations under the License.
           };
         },
 
-        /**
-         * In pages with large diffs, creating the first comment thread can be
-         * slow because nested Polymer elements (particularly
-         * iron-autogrow-textarea) add style elements to the document head,
-         * which, in turn, triggers a reflow on the page. Create a hidden
-         * thread, attach it to the page, and remove it so the stylesheet will
-         * already exist and the user's comment will be quick to load.
-         * @see https://gerrit-review.googlesource.com/c/82213/
-         */
-        _preRenderThread() {
-          const thread = document.createElement('gr-diff-comment-thread');
-          thread.setAttribute('hidden', true);
-          thread.addDraft();
-          const parent = Polymer.dom(this.root);
-          parent.appendChild(thread);
-          Polymer.dom.flush();
-          parent.removeChild(thread);
-        },
-
         /**
          * @return {boolean} whether any of the lines in _groups are longer
          * than SYNTAX_MAX_LINE_LENGTH.