powerpc: drop even more unused Kconfig symbols

When I submitted commit 6805ab6daa2b589fe3242d05ddc47a9dbb0c4eb1
("powerpc: drop unused Kconfig symbols") I apparently failed to notice
that my patch also made PREP_RESIDUAL and PPC_A2_DD2 unused. Drop these
now.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>