Remove Pasteboard::clear, since it is a no-op.

As the comment indicated, this is taken care of on the Chromium side.

BUG=276565

Review URL: https://chromiumcodereview.appspot.com/23173008

git-svn-id: svn://svn.chromium.org/blink/trunk@156523 bbb929c8-8fbe-4397-9dbb-9b2b20218538