src: Revert "nix stdin _readableState.reading"

This reverts 8cee8f54 which was causing stdin to behave strangely on
Windows 8 and 10. The suspected explanation for the issue is that there
might be a race condition occuring when stdin._readableState.reading is
set indirectly through `push('')`.

PR-URL: https://github.com/nodejs/node/pull/3490
Fixes: https://github.com/nodejs/node/issues/2996
Fixes: https://github.com/nodejs/node/issues/2504


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>