Remove (more) unused code from SerializedScriptValue

Following r162708, a few more methods on SSV can be purged.

R=adamk

Review URL: https://codereview.chromium.org/90103002

git-svn-id: svn://svn.chromium.org/blink/trunk@162720 bbb929c8-8fbe-4397-9dbb-9b2b20218538