Purge the portal support code

The original idea here was that we would have an EmulatedInput portal that allows the application to connect directly to the EIS implementation to exchange input events - instead of ping-ponging DBus events through the xdg-desktop-portal as the RemoteDesktop portal requires.

This is no longer accurate, there are suggested PRs open to add RemoteDesktop.ConnectToEIS to achieve the same through the existing RemoteDesktop interface [1] and to add a new InputCapture portal to allow for events to be sent to a libei receiver context [2].

The example EmulatedInput portal is thus superfluous and can be removed from here.

We could switch the ei_setup_backend_portal() code to use RemoteDesktop or InputCapture, depending on the context type, the utility of this is questionable. Interaction with portals is complex, one needs to implement the Session/Request interfaces correctly and in the case of InputCapture also handle the complex zones/pointer barrier setup. libportal does some of this (or it will, anyway) so it's more useful for an application to use libportal and then just pass the received fd to libei.

If there is a future need for this to be handled as part of libei, we can (re)implement this, but for now it's best to just purge all of this.

[1] https://github.com/flatpak/xdg-desktop-portal/pull/762 [2] https://github.com/flatpak/xdg-desktop-portal/pull/714