kbuild: Remove checking for adhoc CONFIG symbols

At this point all listed adhoc CONFIG symbols have been migrated to
Kconfig or removed from the tree or renamed to CFG (or similar). We also
now have CI tests that will error on any new introductions, and
checkpatch.pl also looks. We can now remove these hooks and related
scripts.

Signed-off-by: Tom Rini <trini@konsulko.com>