powerpc/prom_init: Remove linux,stdout-package property

This property was added in 2004 and the only use of it, which was
already inside `#if 0`, was removed a month later.

Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>