readline: remove deprecated methods

This commit removes the deprecated exports getStringWidth(),
stripVTControlCharacters(), and isFullWidthCodePoint(). It also
removes codePointAt() in its entirety, as it was deprecated and
not being used by core.

Refs: https://github.com/nodejs/node/pull/3862
PR-URL: https://github.com/nodejs/node/pull/6423


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>