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 b841d4f (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/[email protected]/

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

cache.h

@@ -1588,8 +1588,6 @@ struct date_mode *date_mode_from_type(enum date_mode_type type);
 
 const char *show_date(timestamp_t time, int timezone, const struct date_mode *mode);
 void show_date_relative(timestamp_t time, struct strbuf *timebuf);
-void show_date_human(timestamp_t time, int tz, const struct timeval *now,
-			struct strbuf *timebuf);
 int parse_date(const char *date, struct strbuf *out);
 int parse_date_basic(const char *date, timestamp_t *timestamp, int *offset);
 int parse_expiry_date(const char *date, timestamp_t *timestamp);