pack-objects: drop unused return value from add_object_entry()

pack-objects: drop unused return value from add_object_entry()

This function returns 0/1 to its caller to tell them whether we actually
added a new entry (or if we considered it redundant). But nobody has
relied on that behavior since 5379a5c5ee (Thin pack generation:
optimization., 2006-04-05).

The extra return does not hurt much, but it is a bit confusing. We have
a sister function, add_object_entry_from_bitmap(), which has the same
return value semantics. That function is about to change to always return
0 (not void, because it must conform to a callback function interface).
So with that change, we'd have two related functions which both return
an "int" but with different semantics.

Let's drop the unused "int" return from add_object_entry() entirely,
which makes it more clear that the two functions have diverged.

Signed-off-by: Jeff King <redacted>
[ps: slightly massaged the commit message]
Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>