um: vector: remove unused len variable/calculation

The len variable is unused, so not needed, remove it.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/vector_kern.c

@@ -712,11 +712,9 @@ static struct vector_device *find_device(int n)
 static int vector_parse(char *str, int *index_out, char **str_out,
 			char **error_out)
 {
-	int n, len, err;
+	int n, err;
 	char *start = str;
 
-	len = strlen(str);
-
 	while ((*str != ':') && (strlen(str) > 1))
 		str++;
 	if (*str != ':') {