powerpc/32: Get rid of sub_reloc_offset()

sub_reloc_offset() has not been used since commit
917f0af9 ("powerpc: Remove arch/ppc and include/asm-ppc") which
removed include/asm-ppc/prom.h.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>