[isolate] Remove dead declaration and dead definition

{Isolate::per_isolate_thread_data_key()} is not even defined, and
{Isolate::isolate_key()} is unused.

R=mstarzinger@chromium.org

Bug: v8:8562
Change-Id: I490989510865903c702158e33621c9990052c2a8
Reviewed-on: https://chromium-review.googlesource.com/c/1425907Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58997}

src/isolate.h

@@ -661,15 +661,6 @@ class Isolate final : private HiddenFactory {
   // If one does not yet exist, no-op.
   void DiscardPerThreadDataForThisThread();
 
-  // Returns the key used to store the pointer to the current isolate.
-  // Used internally for V8 threads that do not execute JavaScript but still
-  // are part of the domain of an isolate (like the context switcher).
-  static base::Thread::LocalStorageKey isolate_key() {
-    return isolate_key_;
-  }
-
-  static base::Thread::LocalStorageKey per_isolate_thread_data_key();
-
   // Mutex for serializing access to break control structures.
   base::RecursiveMutex* break_access() { return &break_access_; }