rpmsg: Remove unused method pointers *send_offchannel

After the previous patch, there are no implementers of the
send_offchannel() and trysend_offchannel() methods.

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/20250429234600.301083-4-linux@treblig.org


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