[cleanup] Remove three more obsolete view singletons.

1. Remove the `NodeConnectionsPanel` singleton.
2. Remove the `ChangesView` singleton together with the unused
   `DiffUILocation` and its revealer.
3. Remove the `IssuesPane` singleton and use the revealer in the
   Elements panel to avoid tight coupling.

Bug: chromium:1379221
Change-Id: I3fcae6aa8c740dd3ded01d01f6855e9aec021c7e
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/5111179
Reviewed-by: Simon Zünd <[email protected]>
Commit-Queue: Benedikt Meurer <[email protected]>
Auto-Submit: Benedikt Meurer <[email protected]>
diff --git a/front_end/entrypoints/node_app/node_app.ts b/front_end/entrypoints/node_app/node_app.ts
index f86f347..e27e34a 100644
--- a/front_end/entrypoints/node_app/node_app.ts
+++ b/front_end/entrypoints/node_app/node_app.ts
@@ -56,7 +56,7 @@
   commandPrompt: i18nLazyString(UIStrings.showConnection),
   order: 0,
   async loadView() {
-    return NodeConnectionsPanel.instance();
+    return new NodeConnectionsPanel();
   },
   tags: [i18nLazyString(UIStrings.node)],
 });