list-objects: drop process_gitlink() function

list-objects: drop process_gitlink() function

Our object graph traversal code has a process_gitlink() function which
we call when we see a gitlink entry. The function does nothing; it was
added in the early days of gitlinks by 6e2f441bd4 (Teach git
list-objects logic to not follow gitlinks, 2007-04-13).

The comment above the function talks about some things we _could_ do.
But in the intervening 15 years, nobody has touched the function, and
the submodule code usually makes its own decisions about when and how to
examine the links. At the generic traversal layer, we can't assume that
the pointed-to commit is available.

Let's drop this placeholder that isn't really helping anything. This
silences some -Wunused-parameter warnings, and also gets rid of a crufty
use of "const unsigned char *" to pass a raw hash value.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>