weston: 5.0.0 -> 6.0.0 (#58522)

https://lists.freedesktop.org/archives/wayland-devel/2019-March/040356.html
https://lists.freedesktop.org/archives/wayland-devel/2019-March/040334.html



* weston: use freerdp2 not legacy
* freerdp_legacy: drop as no longer needed (weston was last user)

Co-Authored-By: Michael Weiss <dev.primeos@gmail.com>