Empty parent gates should be removed

[aatng-gh]

Describe the bug
When the last operation is removed from a group, the group collapses and can't be removed/fixed

To Reproduce
Steps to reproduce the behavior:

1. Drag gate H outside
2. Drag gate RX outside
3. Gate Foo is still visible

Expected behavior
Should remove the group all together if there are no more gates left.

Screenshots

[aatng-gh]

@anpaz A funny coincident fix that already solved this bug