Fix _removeModels regression

Fixes #3693.

This leaves open the question of whether events triggered on the model
during a ‘remove’ listener should also trigger on the model. Just
something to revisit for V2.

```js
col.on('other', (model) => {
// Should this be triggered?
});

col.on('remove', (model) => {
// If the model is really "removed" (we can't `#get` it anymore)
// by the time this listener is called, then I'd argue that this
// shouldn't trigger the 'other' event on the collection...
model.trigger('other');
});
```

test/collection.js

@@ -298,12 +298,13 @@
     deepEqual(col.pluck('id'), [1, 2, 3]);
   });
 
-  test("remove", 10, function() {
+  test("remove", 11, function() {
     var removed = null;
     var result = null;
     col.on('remove', function(model, col, options) {
       removed = model.get('label');
       equal(options.index, 3);
+      equal(col.get(model), undefined, '#3693: model cannot be fetched from collection');
     });
     result = col.remove(d);
     equal(removed, 'd');