repack: remove 'remove_redundant_pack' from the builtin

repack: remove 'remove_redundant_pack' from the builtin

Extract "remove_redundant_pack()" as generic repack-related
functionality by moving its implementation to the repack.[ch]
compilation unit.

This is a prerequisite to moving the "existing_packs" API, which is one
of the callers of this function. (The remaining caller in the pack
geometry code will eventually move to its own compilation unit as well,
and will likewise rely on this function.)

While moving it over, prefix the function name with "repack_" to
indicate that it belongs to the repack-subsystem.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>