src: remove misleading flag in TwoByteValue

String::REPLACE_INVALID_UTF8 is only applied in V8's
String::WriteUtf8() (i.e. Utf8Value).

PR-URL: https://github.com/nodejs/node/pull/11436


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>