Remove unused code from SerializedScriptValue

Hopefully the first in a series of SSV cleanups. This is truly dead, uncalled
code as determined by an internal tool (scythe)

BUG=

Review URL: https://codereview.chromium.org/83443004

git-svn-id: svn://svn.chromium.org/blink/trunk@162578 bbb929c8-8fbe-4397-9dbb-9b2b20218538