macos: future-proof MakefileProto.DARWIN.in

if new cases show up which match the current ones, they just work.
darwin60 and older were already killed from the tree, kill here also

Reviewed-on: http://gerrit.openafs.org/7782
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit e6aa586a996e32b3def4b8108815730fcfbf1463)

Change-Id: I66bb6eb54856a37d3ff14cf455d90c482443a00e
Reviewed-on: http://gerrit.openafs.org/8912
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>