DOM: retire runtime flag for FocusOptions

Ability to Prevent scrolling in HTMLElement.focus() shipped in M64
https://chromium-review.googlesource.com/731225
https://www.chromestatus.com/feature/5745122025144320

BUG=734166

Change-Id: Ia2f8ebe29c553069e4893f7b474bd21b4b1f29a2
Reviewed-on: https://chromium-review.googlesource.com/1163056Reviewed-by: Ella Ge <eirage@chromium.org>
Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581154}

third_party/blink/renderer/core/html/focus_options.idl

 dictionary FocusOptions {
-    [RuntimeEnabled=FocusOptions]boolean preventScroll = false;
+    boolean preventScroll = false;
 };

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -496,10 +496,6 @@
       name: "FileSystem",
       status: "stable",
     },
-    {
-      name: "FocusOptions",
-      status: "stable",
-    },
     {
       name: "FontCacheScaling",
       status: "test",