Remove a check added by #1393 trying to fix #1338. The check only accounted for MSYS2 environments that used GCC. #1135 was later merged that added detection for Windows consoles, which would have also addressed the issue. I confirmed that click.echo and click.progressbar continued to work with the MSYS2 check removed, for msys2 and cygwin environments (which Python sees as posix) and mingw-w64 (which Python sees as Windows). fixes #2355

I also noticed that there was a check for Google App Engine. This check seems to be from when GAE still supported Python 2, and provided a special local dev server environment. The GAE standard environment sets different env vars now, and the local dev server appears to just run Python in Windows still (and is de-emphasized over running the usual flask dev server). I've removed that check as well.