soc: intel_adsp: Remove old support for cAVS 2.5 "alternate boot"

The linker script (and a little bit of SOF) still has support for an
older mechanism for bootstrapping secondary cores by copying code into
lp-sram from a "manifest" emitted by the linker.  This actually never
worked in Zephyr, and we've implemented a different scheme that uses a
small runtime-copied trampoline instead.

Remove.

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