prune_refs(): use delete_refs()

The old version just looped over the references to delete, calling
delete_ref() on each one. But that has quadratic behavior, because
each call to delete_ref() might have to rewrite the packed-refs file.
This can be very expensive in a repository with a large number of
references. In some (admittedly extreme) repositories, we've seen
cases where the ref-pruning part of fetch takes multiple tens of
minutes.

Instead call delete_refs(), which (aside from being less code) has the
optimization that it only rewrites the packed-refs file a single time.

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

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