xdiff: drop xpparam_t parameter from histogram cmp_recs()

xdiff: drop xpparam_t parameter from histogram cmp_recs()

Since 663c5ad035 (diff histogram: intern strings, 2021-11-17), our
cmp_recs() does not call xdl_recmatch(), and thus no longer needs an
xpparam_t struct from which to get the flags. We can drop the unused
parameter from the function, as well as the macro which wraps it.

There's no functional change here; it's just simplifying things (and
making -Wunused-parameter happier).

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