refspec: remove refspec_item_init_or_die()

There are two callers of this function, which ensures that a dispatched call to refspec_item_init() does not fail. In the following commit, we're going to add fetch/push-specific variants of refspec_item_init(), which will turn one function into two. To avoid introducing yet another pair of new functions (such as refspec_item_init_push_or_die() and refspec_item_init_fetch_or_die()), let's remove the thin wrapper entirely. This duplicates a single line of code among two callers, but thins the refspec.h API by one function, and prevents introducing two more in the following commit. Note that we still have a trailing Boolean argument in the function `refspec_item_init()`. The following commit will address this. Signed-off-by: Taylor Blau <me@ttaylorr.com> Acked-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>