Retire LongTaskObserver runtime flag

The LongTaskObserver API enables applications to detect presence of
“long tasks” that monopolize the UI thread for extended periods of
time and block other critical tasks from being executed.
https://codereview.chromium.org/2685183002
https://www.chromestatus.com/feature/5738471184400384

BUG=688439

Change-Id: Ibfa8745abd6ff4a670fa94859b7d211d10ab489c
Reviewed-on: https://chromium-review.googlesource.com/1166621Reviewed-by: Shubhie Panicker <panicker@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582110}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -637,10 +637,6 @@
       name: "LazyParseCSS",
       status: "experimental",
     },
-    {
-      name: "LongTaskObserver",
-      status: "stable",
-    },
     {
       name: "LongTaskV2",
     },