powerpc/boot: Drop unused dummy.c

The last use of dummy.c was dropped in commit 2bf11819 ("[POWERPC]
Create a "wrapper" script and use it in arch/powerpc/boot").

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220819110556.434970-1-mpe@ellerman.id.au