Remove more dead code in Editor for copying URLs.

Formerly, ContextMenuController referenced this method but it was removed
in https://src.chromium.org/viewvc/blink?view=rev&revision=150375 since
this is handled by the embedder in Chrome/Blink.

Review URL: https://chromiumcodereview.appspot.com/24469003

git-svn-id: svn://svn.chromium.org/blink/trunk@158360 bbb929c8-8fbe-4397-9dbb-9b2b20218538