powerpc: Remove 'struct ppc64_opd_entry'

'struct ppc64_opd_entry' doesn't belong to uapi/asm/elf.h

It was initially in module_64.c and commit 2d291e90 ("Fix compile
failure with non modular builds") moved it into asm/elf.h

But it was by mistake added outside of __KERNEL__ section,
therefore commit c3617f72 ("UAPI: (Scripted) Disintegrate
arch/powerpc/include/asm") moved it to uapi/asm/elf.h

Now that it is not used anymore by the kernel, remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c309ccee65ec2e3802df7a7fe761d0a298584809.1644928018.git.christophe.leroy@csgroup.eu