powerpc: Drop CONFIG_WALNUT and other related dead code

CONFIG_WALNUT was dropped in June 2017 in:
commit 98f705c9 ("powerpc: remove 4xx support")

While at it, the related CONFIG_MACH_SPECIFIC and the have_of
and _machine variables are unused as well, so drop them too.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>