cache.h: remove always unused show_date_human() declaration

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>