hash.h: drop unsafe_ function variants

hash.h: drop unsafe_ function variants

Now that all callers have been converted from:

    the_hash_algo->unsafe_init_fn();

to

    unsafe_hash_algo(the_hash_algo)->init_fn();

and similar, we can remove the scaffolding for the unsafe_ function
variants and force callers to use the new unsafe_hash_algo() mechanic
instead.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>