Revert "Add experimental newest-first issue thread" (#5460)

This is actually bad. Glad it was under experiments.

ui/src/pages/InstanceExperimentalSettings.tsx

@@ -205,7 +205,6 @@ export function InstanceExperimentalSettings() {
 
   const enableEnvironments = experimentalQuery.data?.enableEnvironments === true;
   const enableIsolatedWorkspaces = experimentalQuery.data?.enableIsolatedWorkspaces === true;
-  const enableNewestFirstIssueThread = experimentalQuery.data?.enableNewestFirstIssueThread === true;
   const autoRestartDevServerWhenIdle = experimentalQuery.data?.autoRestartDevServerWhenIdle === true;
   const enableIssueGraphLivenessAutoRecovery =
     experimentalQuery.data?.enableIssueGraphLivenessAutoRecovery === true;
@@ -299,25 +298,6 @@ export function InstanceExperimentalSettings() {
         </div>
       </section>
 
-      <section className="rounded-xl border border-border bg-card p-5">
-        <div className="flex items-start justify-between gap-4">
-          <div className="space-y-1.5">
-            <h2 className="text-sm font-semibold">Enable Newest-First Issue Thread</h2>
-            <p className="max-w-2xl text-sm text-muted-foreground">
-              Show issue comments and messages with the newest activity first, move the jump control to the bottom of
-              the page, and surface plain comment timestamps in the header area.
-            </p>
-          </div>
-          <ToggleSwitch
-            checked={enableNewestFirstIssueThread}
-            onCheckedChange={() =>
-              toggleMutation.mutate({ enableNewestFirstIssueThread: !enableNewestFirstIssueThread })}
-            disabled={toggleMutation.isPending}
-            aria-label="Toggle newest-first issue thread experimental setting"
-          />
-        </div>
-      </section>
-
       <section className="rounded-xl border border-border bg-card p-5">
         <div className="flex items-start justify-between gap-4">
           <div className="space-y-1.5">