Remove unused Internals::setShadowPseudoId

This hasn't been used for over a year.

R=alancutter

Review URL: https://codereview.chromium.org/595233002

git-svn-id: svn://svn.chromium.org/blink/trunk@182667 bbb929c8-8fbe-4397-9dbb-9b2b20218538