powerpc/configs: Drop old symbols from ppc6xx_defconfig

ppc6xx_defconfig refers to quite a few symbols that no longer exist,
as reported by scripts/checkkconfigsymbols.py, remove them.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724131728.1643966-1-mpe@ellerman.id.au