remoteproc: k3: Remove remote processor mailbox ping

remoteproc: k3: Remove remote processor mailbox ping

As of today, pinging the remote processor during k3_rproc_request_mbox()
does not have any functional effect. This behavior was originally based on
the OMAP remoteproc driver, where the idea was to send messages such as
suspend requests (among others) to the remote processor, but this was
never upstreamed.

Currently, the ping message has no effect in upstream usage and causes an
unread message to remain in the mailbox, which ultimately prevents the
system from entering suspend mode:

Freezing remaining freezable tasks completed (elapsed 0.001 seconds)
printk: Suspending console(s) (use no_console_suspend to debug)
omap-mailbox 29000000.mailbox: fifo 1 has unexpected unread messages
omap-mailbox 29000000.mailbox: PM: dpm_run_callback(): platform_pm_suspend returns -16
omap-mailbox 29000000.mailbox: PM: failed to suspend: error -16

The ping is only replied if the remote core firmware is capable of doing
it, otherwise the unread message stays into the mailbox.

Remove the ping and fix the suspend issue.

Suggested-by: Andrew Davis <afd@ti.com>
Signed-off-by: Hiago De Franco <hiago.franco@toradex.com>
Acked-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20250806-v1-fix-am62-hmp-suspend-v1-1-1c4a81bb5dde@toradex.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>