windows/README: Remove unsupported Python instructions for Cygwin.
windows/README: Remove unsupported Python instructions for Cygwin.

It's not possible anymore to build MicroPython on Cygwin using a
standard Windows installation of Python so don't advertise that.
Specifically: preprocessing in makeqstrdefs.py fails on the subprocess
call with 'gcc: fatal error: no input files' because one of the flags
contains double quotes and that somehow messes up the commandline.

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