rpmsg: core: Remove deadcode

rpmsg_send_offchannel() and rpmsg_trysend_offchannel() have been
unused since they were added in 2011's
commit bcabbcca ("rpmsg: add virtio-based remote processor messaging
bus")

Remove them and associated docs.

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/20250429234600.301083-2-linux@treblig.org


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>