refs/files: remove duplicate duplicates check

refs/files: remove duplicate duplicates check

Within the files reference backend's transaction's 'finish' phase, a
verification step is currently performed wherein the refnames list is
sorted and examined for multiple updates targeting the same refname.

It has been observed that this verification is redundant, as an
identical check is already executed during the transaction's 'prepare'
stage. Since the refnames list remains unmodified following the
'prepare' stage, this secondary verification can be safely eliminated.

The duplicate check has been removed accordingly, and the
`ref_update_reject_duplicates()` function has been marked as static, as
its usage is now confined to 'refs.c'.

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>