pack-objects: get rid of reuse_cached_pack

pack-objects: get rid of reuse_cached_pack

This capability is practically never useful, and therefore never tested,
because it is fairly unlikely that the requested pack will be already
available.  Furthermore it is of little gain over the ability to reuse
existing pack data.

In fact the ability to change delta type on the fly when reusing delta
data is a nice thing that has almost no cost and allows greater backward
compatibility with a client's capabilities than if the client is blindly
sent a whole pack without any discrimination.

And this "feature" is simply in the way of other cleanups.
Let's get rid of it.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>