Function do_not_prune() was redundantly checking REF_ISSYMREF, which
was already tested at the top of pack_one_ref(), so remove that check.
And the rest was trivial, so inline the function.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>