Replace gr-diff-comment-thread-group with div

This element is now nothing but an empty container, so I think it's
better to just use a div.

Change-Id: I58da47854d2c50608f9e217e7a12e30aad9aca98
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
index b258455..80ca01d 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -36,6 +36,11 @@
       :host(.no-left) .sideBySide ::content .right:not([data-value]) + td {
         display: none;
       }
+      ::slotted(*) .thread-group {
+        display: block;
+        max-width: var(--content-width, 80ch);
+        white-space: normal;
+      }
       .diffContainer {
         display: flex;
         font-family: var(--monospace-font-family);