pack-objects: clean up list sorting

pack-objects: clean up list sorting

Get rid of sort_comparator() as it impose a run time double indirect
function call for little compile time type checking gain.

Also get rid of create_sorted_list() as it only has one user which would
as well be just fine doing its sorting locally.  Eventually the list of
deltifiable objects might be shorter than the whole object list.

Signed-off-by: Nicolas Pitre <redacted>
Signed-off-by: Junio C Hamano <redacted>