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>