powerpc: remove meaningless KBUILD_ARFLAGS addition

The KBUILD_ARFLAGS addition in arch/powerpc/Makefile has never worked
in a useful way because it is always overridden by the following code
in the top Makefile:

  # use the deterministic mode of AR if available
  KBUILD_ARFLAGS := $(call ar-option,D)

The code in the top Makefile was added in 2011, by commit 40df759e
("kbuild: Fix build with binutils <= 2.19").

The KBUILD_ARFLAGS addition for ppc has always been dead code from the
beginning.

Nobody has reported a problem since 43c9127d ("powerpc: Add option
to use thin archives"), so this code was unneeded.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190713032106.8509-1-yamada.masahiro@socionext.com