buffer: don't set `kNoZeroFill` flag in allocUnsafe

If `kNoZeroFill` is set here, it won't be reset in case of
pooled allocation. In case of "slow" allocation it will be
set later anyway.

Fixes: https://github.com/nodejs/node/issues/6006
PR-URL: https://github.com/nodejs/node/pull/6007


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>