net/xrdp: Relax dependency on x11-drivers/xorgxrdp

Now x11-drivers/xorgxrdp provides the default xorg flavor and the xlibre
flavor.

To avoid tying xrdp to a specific xorgxrdp flavor, the runtime
dependency on xorgxrdp has been removed. In addition, pkg-message has
been updated to instruct users to install either flavor of xorgxrdp
according to their environment.