grep.c: remove redundant line of code

Signed-off-by: Angus Hammond <angusgh@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -289,7 +289,7 @@ static struct grep_expr *prep_header_patterns(struct grep_opt *opt)
 
 	if (!opt->header_list)
 		return NULL;
-	p = opt->header_list;
+
 	for (p = opt->header_list; p; p = p->next) {
 		if (p->token != GREP_PATTERN_HEAD)
 			die("bug: a non-header pattern in grep header list.");