hex: drop sha1_to_hex_r()

hex: drop sha1_to_hex_r()

There are no callers left; everybody uses oid_to_hex_r() or
hash_to_hex_algop_r(). This used to actually be the underlying
implementation for oid_to_hex_r(), but that's no longer the case since
47edb64997 (hex: introduce functions to print arbitrary hashes,
2018-11-14).

Let's get rid of it to de-clutter and to make sure nobody uses it.
Likewise we can drop the coccinelle rules that mention it, since the
compiler will make it quite clear that the code does not work.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>