Makefile: clean .depend dirs under COMPUTE_HEADER_DEPENDENCIES != yes

Fix a logic error in dfea575017d (Makefile: lazily compute header dependencies, 2010-01-26) where we'd make whether we cleaned the .depend dirs contingent on the currently configured COMPUTE_HEADER_DEPENDENCIES value. Before this running e.g.: make COMPUTE_HEADER_DEPENDENCIES=yes grep.o make COMPUTE_HEADER_DEPENDENCIES=no clean Would leave behind the .depend directory, now it'll be removed. Normally we'd need to use another variable, but in this case there's no other uses of $(dep_dirs), as opposed to $(dep_args) which is used as an argument to $(CC). So just deleting this line makes everything work correctly. See http://lore.kernel.org/git/xmqqmto48ufz.fsf@gitster.g for a report about this issue. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>