csum-file: drop `hashfd_throughput()`

csum-file: drop `hashfd_throughput()`

The `hashfd_throughput()` function is used by a single callsite in
git-pack-objects(1). In contrast to `hashfd()`, this function uses a
progress meter to measure throughput and a smaller buffer length so that
the progress meter can provide more granular metrics.

We're going to change that caller in the next commit to be a bit more
specific to packing objects. As such, `hashfd_throughput()` will be a
somewhat unfitting mechanism for any potential new callers.

Drop the function and replace it with a call to `hashfd_ext()`.

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