rpmsg: virtio: Remove uncallable offchannel functions

rpmsg: virtio: Remove uncallable offchannel functions

The virtio_rpmsg_send_offchannel() and virtio_rpmsg_trysend_offchannel()
are wired up to the send_offchannel and trysend_offchannel members
of rpmsg_endpoint_ops, the only dereference of those was by
the unused, and now removed rpmsg_send_offchannel() and
rpmsg_trysend_offchannel() functions.

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20250429234600.301083-3-linux@treblig.org
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>