[POWERPC] Rip out the existing powerpc msi stubs

Rip out the existing powerpc msi stubs. These were the start of an
implementation based on ppc_md calls, but were never used in mainline.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>