xdiff: drop unused parameter in def_ff()

xdiff: drop unused parameter in def_ff()

The def_ff() function is the default "find_func" for finding hunk
headers. It has never used its "priv" argument since it was introduced
in f258475a6e (Per-path attribute based hunk header selection.,
2007-07-06). But back then we used a function pointer to switch between
a caller-provided function and the default, so the two had to conform to
the same interface.

In ff2981f724 (xdiff: factor out match_func_rec(), 2016-05-28), that
pointer indirection went away in favor of code which directly calls
either of the two functions. So there's no need for def_ff() to retain
this unused parameter.

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