Remove PrivateScriptInterface

We changed the design of Blink-in-JS, and we no longer need an PrivateScriptInterface,
which can support only static methods.

BUG=341031

Review URL: https://codereview.chromium.org/370563002

git-svn-id: svn://svn.chromium.org/blink/trunk@177478 bbb929c8-8fbe-4397-9dbb-9b2b20218538