x11: Drop support for multi-head

The main reason to drop multi-head support is that it has been simply
unmaintained for many many years. When implementing a feature, we don't
even bother checking if multi-head is broken, KCMs don't handle
multihead, window management features are written for Xinerama. KWin
is optimized for Xinerama-like operation mode in general, which is
provided out of the box.

If you use multihead for esoteric gpu stuff, consider using kwin_wayland!