merge-ort: drop unused "opt" parameter from merge_check_renames_reusable()

merge-ort: drop unused "opt" parameter from merge_check_renames_reusable()

The merge_options parameter has never been used since the function was
introduced in 64aceb6d73 (merge-ort: add code to check for whether
cached renames can be reused, 2021-05-20). In theory some merge options
might impact our decisions here, but that has never been the case so
far.

Let's drop it to appease -Wunused-parameter; it would be easy to add
back later if we need to (there is only one caller).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>