Since Wayland 1.15, it is now possible to use absolute paths in
"WAYLAND_DISPLAY".
In that scenario, having a valid "XDG_RUNTIME_DIR" is not a requirement
anymore.
For this reason we remove the "XDG_RUNTIME_DIR" check and we let
`wl_display_connect()` decide if our environment is correct.
Signed-off-by: Ludovico de Nittis <ludovico.denittis@collabora.com>