lib: sbi_ipi: Drop unnecessary ipi_process check

sbi_ipi_event_create() disallows registering an IPI event with a NULL
.process callback, so the function pointer will never be NULL here.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Xiang W <wxjstz@126.com>