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

v2:

• Remove rootless include paths
• Remove windowswmproto from meson.build