drivers/ipm: Remove intel_adsp_mailbox driver

This is dead code.  It's based on the cAVS "IPC" mechanism to allow
communication to and from the host CPU.  But there is no test rig in
the Zephyr tree for the protocol defined.  And in fact the only
Zephyr-based user of the IPC mechanism (Sound Open Firmware) has its
own IPC driver and speaks its own protocol with the host kernel.  That
driver needs to migrate into Zephyr soon and this legacy bit is just
confusing.

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