hash: drop (mostly) unused `is_empty_{blob,tree}_sha1()` functions

hash: drop (mostly) unused `is_empty_{blob,tree}_sha1()` functions

The functions `is_empty_{blob,tree}_sha1()` are mostly unused, except
for a single callsite in "read-cache.c". Most callsites have long since
been converted to use the equivalents that accept a `struct object_id`
instead of a string.

Adapt the remaining callsite and drop those functions.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>