Reverts flutter/flutter#136771
Initiated by: gspencergoog
This change reverts the following previous change:
Original Description:
## Description

This adds code to make sure that grandchildren are removed from the `focusedChild` of a scope when the child is detached.

## Related Issues
 - Fixes https://github.com/flutter/flutter/issues/136758

## Tests
 - Added regression test.