prune_remote(): use delete_refs()

prune_remote(): use delete_refs()

This slightly changes how errors are reported. The old and new code
both report errors that come from repack_without_refs() the same way.
But if an error occurs within delete_ref(), the old version only
emitted an error within delete_ref() without further comment. The new
version (in delete_refs()) still emits that error, but then follows it
up with

    error(_("could not remove reference %s"), refname)

The "could not remove reference" error originally came from a similar
message in remove_branches() (from builtin/remote.c).

This is an improvement, because the error from delete_ref() (which
usually comes from ref_transaction_commit()) can be pretty low-level,
like

    Cannot lock ref '%s': unable to resolve reference %s: %s

where the last "%s" is the original strerror.

In any case, I don't think we need to sweat the details too much,
because these errors should only ever be seen in the case of a broken
repository or a race between two processes; i.e., only in pretty rare
and anomalous situations.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>