powerpc: drop even more unused Kconfig symbols

When I submitted commit 6805ab6d
("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>

arch/powerpc/platforms/wsp/Kconfig

@@ -28,7 +28,3 @@ config PPC_CHROMA
 	default y
 
 endmenu
-
-config PPC_A2_DD2
-	bool "Support for DD2 based A2/WSP systems"
-	depends on PPC_A2