appveyor: Revert commits adding Cygwin support.

This reverts commits 00ad77b9f683e561b1ac45fbb89eb2bafe45c8c6 and
5334dafee265d78abdfcf30e2c693e0791bfecf5.

This avoids Appveyor build breakage due to Cygwin, but more importantly,
there are several problems with these patches, as highlighted to my
recent mesa-dev mail.  So better to revert for now, and pursue Cygwin
support after these have been address.
3 files changed