Remove st != NULL checks from recursor functions

In the description of 3111908b03, it says
that the functions must be able to handle st being NULL, but recurse
always passes a valid pointer. The only function that was ever passed
NULL was rm(), but this was changed to go through recurse in
2f4ab52739, so now the checks are
pointless.

libutil/rm.c

@@ -13,7 +13,7 @@ int rm_status = 0;
 void
 rm(const char *path, struct stat *st, void *data, struct recursor *r)
 {
-	if (!r->maxdepth && st && S_ISDIR(st->st_mode)) {
+	if (!r->maxdepth && S_ISDIR(st->st_mode)) {
 		recurse(path, NULL, r);
 
 		if (rmdir(path) < 0) {