ref-filter: remove unused ref_format member

use_rest was added in b9dee075eb (ref-filter: add %(rest) atom,
2021-07-26) but was never used. As far as I can tell it was used in a
later patch that was submitted to the mailing list but never applied.

Signed-off-by: Øystein Walle <oystwa@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -582,7 +582,6 @@ static int rest_atom_parser(struct ref_format *format, struct used_atom *atom,
 {
 	if (arg)
 		return err_no_arg(err, "rest");
-	format->use_rest = 1;
 	return 0;
 }
 

ref-filter.h

@@ -75,7 +75,6 @@ struct ref_format {
 	const char *format;
 	const char *rest;
 	int quote_style;
-	int use_rest;
 	int use_color;
 
 	/* Internal state to ref-filter */