fs: undeprecate exists() and existsSync()

fs.exists() and fs.existsSync() were deprecated in #103. This
commit removes the deprecation message, in order to stay more
in sync with joyent/node for the io.js 1.0.0 release.

Fixes: https://github.com/iojs/io.js/issues/257
PR-URL: https://github.com/iojs/io.js/pull/307


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Bert Belder <bertbelder@gmail.com>