builtin-mailinfo.c: fix confusing internal API to mailinfo()

It fed two arguments to override the corresponding global variables,
but the caller always assigned the values to the global variables
first and then passed those global variables to this function.

Stop pretending to be a proper API to confuse people.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-mailinfo.c

@@ -885,12 +885,9 @@ static void handle_info(void)
 	fprintf(fout, "\n");
 }
 
-static int mailinfo(FILE *in, FILE *out, int ks, const char *encoding,
-		    const char *msg, const char *patch)
+static int mailinfo(FILE *in, FILE *out, const char *msg, const char *patch)
 {
 	int peek;
-	keep_subject = ks;
-	metainfo_charset = encoding;
 	fin = in;
 	fout = out;
 
@@ -956,5 +953,5 @@ int cmd_mailinfo(int argc, const char **argv, const char *prefix)
 	if (argc != 3)
 		usage(mailinfo_usage);
 
-	return !!mailinfo(stdin, stdout, keep_subject, metainfo_charset, argv[1], argv[2]);
+	return !!mailinfo(stdin, stdout, argv[1], argv[2]);
 }