fs: remove fs.read's string interface

It is a maintenance burden that was removed from the docs in 2010
(c93e0aaf) and runtime-deprecated in v6.0
(1124de2d).

PR-URL: https://github.com/nodejs/node/pull/9683


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>