m68k: defconfig: Update defconfigs for v6.9-rc1

  - Enable trimming of unused exported kernel symbols,
  - Drop CONFIG_IP_NF_ARPTABLES=m (auto-enabled since commit
    4654467d ("netfilter: arptables: allow xtables-nft only
    builds")),
  - Drop CONFIG_STRING_SELFTEST=m (replaced by auto-modular
    CONFIG_STRING_KUNIT_TEST in commit 29d85688 ("string:
    Convert selftest to KUnit")),
  - Drop CONFIG_TEST_STRING_HELPERS=m (replaced by auto-modular
    CONFIG_STRING_HELPERS_KUNIT_TEST in commit fb57550f
    ("string: Convert helpers selftest to KUnit")).

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/e17b3ac60832a3ff92d25d1a05bf814e8f15d0c5.1711475325.git.geert@linux-m68k.org