media: venus: Remove unused hfi_core_ping()

hfi_core_ping() was added by 2017's
commit 09c2845e ("[media] media: venus: hfi: add Host Firmware
Interface (HFI)")

but has remained unused.

Remove it.

It was the only caller of the ->core_ping member of hfi_ops,
so remove it, and the venus_core_ping that it pointed to.

Note I've left pky_sys_ping which seems to be the lowest level
definition of the command.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Stanimir Varbanov <stanimir.k.varbanov@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>