Fall back to using pk/ui from orig table if new table del index uses cols that no longer exist in orig table.

t/pt-online-schema-change/samples/del-trg-bug-1103672.sql

@@ -0,0 +1,20 @@
+drop database if exists test;
+create database test;
+use test;
+
+CREATE TABLE `t1` (
+  `id` int(10) unsigned NOT NULL,
+  `x` char(3) DEFAULT NULL,
+  PRIMARY KEY (`id`)
+) ENGINE=InnoDB;
+
+INSERT INTO t1 VALUES
+  (1, 'a'),
+  (2, 'b'),
+  (3, 'c'),
+  (4, 'd'),
+  (5, 'f'),
+  (6, 'g'),
+  (7, 'h'),
+  (8, 'i'),
+  (9, 'j');