selftests/ipc: Remove unused variables

Delete variables "msg" and "pid" that have never been used.

Link: https://lore.kernel.org/r/20241202045827.4704-1-zhangjiao2@cmss.chinamobile.com


Signed-off-by: zhang jiao <zhangjiao2@cmss.chinamobile.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>