hw/xwin: Remove mwextwm mode
This has always been described as 'experimental'
We don't think this has any users: This mode has been disabled in Cygwin packages since March 2016. We've never provided the xwinwm WM for x86_64 Cygwin. No one has even asked where the option has gone.
This leaves XQuartz as the only user of the rootless extension.
- Remove --enable-windowswm configure option
- Remove multiwindowextwm stuff from Makefiles
- Remove -mwextwm option
- Remove -mwextwm from man-page and help
- Un-ifdef XWIN_MULTIWINDOWEXTWM
- Remove rootless include paths
- Remove windowswmproto from meson.build