cache.h: remove always unused show_date_human() declaration
There has never been a show_date_human() function on the "master"
branch in git.git. This declaration was added in
b841d4ff438 (Add
`human` format to test-tool, 2019-01-28).
A look at the ML history reveals that it was leftover cruft from an
earlier version of that commit[1].
1. https://lore.kernel.org/git/
20190118061805.19086-5-ischis2@cox.net/
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>