kbuild: clean up scripts/headers.sh

'drop' variable is unused.

'ppc' and 'sparc64' directories don't exist in arch/,
and I think their headers can be well exported now, so
just remove them.

Signed-off-by: WANG Cong <amwang@redhat.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>