Remove ScriptState

Now it's no longer used!
I'm planning to rename NewScriptState to ScriptState in a follow-up.

BUG=357144

Review URL: https://codereview.chromium.org/258713009

git-svn-id: svn://svn.chromium.org/blink/trunk@172724 bbb929c8-8fbe-4397-9dbb-9b2b20218538