Async Clipboard: Remove extra system_clipboard() accessor function.

Remove ClipboardPromise::system_clipboard(), which was added in
https://crrev.com/c/1877532, but doesn't seem to be called at all.
Refactoring change, with no functional changes intended.

Change-Id: Ie63574cb279e3df1c05b3446202085b72e1a455f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033976
Auto-Submit: Darwin Huang <huangdarwin@chromium.org>
Commit-Queue: Jarryd Goodman <jarrydg@chromium.org>
Reviewed-by: Jarryd Goodman <jarrydg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737996}

third_party/blink/renderer/modules/clipboard/clipboard_promise.h

@@ -47,8 +47,6 @@ class ClipboardPromise final : public GarbageCollected<ClipboardPromise>,
 
   void Trace(blink::Visitor*) override;
 
-  SystemClipboard* system_clipboard() { return system_clipboard_; }
-
  private:
   // Called to begin writing a type.
   void StartWriteRepresentation();