ServiceWorker: Remove unused ServiceWorkerVersion::SkipWaitingFromDevTools() declaration

This function declaration was added by the following CL, but wasn't
implemented:
https://codereview.chromium.org/1951853004

Bug: n/a
Change-Id: I73657333974bca8255c40db94825660f1c6af949
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2215151
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Auto-Submit: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772158}

content/browser/service_worker/service_worker_version.h

@@ -270,9 +270,6 @@ class CONTENT_EXPORT ServiceWorkerVersion
   // subresources).
   bool OnRequestTermination();
 
-  // Skips waiting and forces this version to become activated.
-  void SkipWaitingFromDevTools();
-
   // Schedules an update to be run 'soon'.
   void ScheduleUpdate();