Fix _removeModels regression

[jridgewell]

Fixes #3693.

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

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');
});

[akre54]

All events maybe not, but a remove definitely should.

[jridgewell]

I think this warrants a v1.2.4?

[akre54]

Want to add a test that col.on('other', ...) doesn't trigger?

[jridgewell]

It's in #3806, but I'm waiting till v2.