refs/files: remove redundant check in split_symref_update()

refs/files: remove redundant check in split_symref_update()

In `split_symref_update()`, there were two checks for duplicate
refnames:

  - At the start, `string_list_has_string()` ensures the refname is not
    already in `affected_refnames`, preventing duplicates from being
    added.

  - After adding the refname, another check verifies whether the newly
    inserted item has a `util` value.

The second check is unnecessary because the first one guarantees that
`string_list_insert()` will never encounter a preexisting entry.

The `item->util` field is assigned to validate that a rename doesn't
already exist in the list. The validation is done after the first check.
As this check is removed, clean up the validation and the assignment of
this field in `split_head_update()` and `files_transaction_prepare()`.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>