cache.h: remove unused "the_index" compat macros
The "active_alloc" macro added in
228e94f9357 (Move index-related
variables into a structure., 2007-04-01) has not been used since
4aab5b46f44 (Make read-cache.c "the_index" free., 2007-04-01). Let's
remove it.
The rest of these are likewise unused, so let's not keep them
around. E.g.
12cd0bf9b02 (dir: stop using the index compatibility
macros, 2017-05-05) is the last use of "cache_dir_exists".
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>