win32: remove deprecated -mno-cygwin options from env.sh

win32: remove deprecated -mno-cygwin options from env.sh

This removes the use of -mno-cygwin from our generated env.sh for cross
compiling with mingw.

I don't know that anyone is building clutter under cygwin, and our
BuildingClutterOnWindows wiki page only describes building with msys not
cygwin so I think its fair to assume that this build configuration is
untested and thus not supported by us currently. Since the -mno-cygwin
option is deprecated it could well be that there is a better
cross-compilation solution available for cygwin these days if you want
to build programs that don't depend on cygwin libraries.