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.

ports/windows/README.md

@@ -25,8 +25,7 @@ Install Cygwin, then install following packages using Cygwin's setup.exe:
 * mingw64-i686-gcc-core
 * mingw64-x86_64-gcc-core
 * make
-
+* python3
-Also install the python3 package, or install Python globally for Windows (see below).
 
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