Remove CYGWIN=tty for Windows 7 SP1

This fixes #422. I believe the cause of the issue is that cygwin changed so that it no longer supported `CYGWIN=tty`.

Whenever I would ssh into the machine, it would complain about `CYGWIN=tty` no longer being supported. This change makes the message go away.

Presumably the same change needs to be run on all the Windows 7 definitions, but I've only tested it with this one.