Fix n/p shortcuts for new diff

Firing `render-start` without `render-content` later has negative
effects on the diff cursor. It expects to get both events, because it
wants to handle scroll events differently while rendering is still in
progress.

It is not clear why we were firing `render-start` both in gr-diff and
in gr-diff-element. One should be enough. And gr-diff-element actually
makes sure to fire both.

Release-Notes: skip
Google-Bug-Id: b/301485781
Change-Id: I100bb10ff09d80a4da7f328925566bc1e69a2ff7

polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts

diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts
index 3696f99..58cad19 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts
@@ -431,7 +431,6 @@
   }
 
   override render() {
-    fire(this, 'render-start', {});
     return html`<gr-diff-element></gr-diff-element>`;
   }