soc/intel_adsp: Remove "firmware ready" message sending

This IPC protocol is designed to tell the host driver that the audio
firmware is ready.  It's not used within the Zephyr in-tree test code,
which does not run under the control of a host driver.  And SOF
already does this on its own, the Zephyr attempt to do it first (and
incorrectly) confuses the driver IPC.

Just remove it.  This is clearly application code, not platform code.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>