[api] Remove experimental tag for PromiseHooks

Bug: v8:4643
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Id82365d20830e5efd33a8d066f5aab4b999807d7
Reviewed-on: https://chromium-review.googlesource.com/611180Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47297}

include/v8.h

@@ -7404,8 +7404,8 @@ class V8_EXPORT Isolate {
           DeprecatedCallCompletedCallback callback));
 
   /**
-   * Experimental: Set the PromiseHook callback for various promise
-   * lifecycle events.
+   * Set the PromiseHook callback for various promise lifecycle
+   * events.
    */
   void SetPromiseHook(PromiseHook hook);