[Bug] Deleting GCP mixes up remaining GCPs

[mradamcox]

Describe the problem If you are georeferencing an item that already has GCPs stored (i.e. editing existing georeferencing) and you delete one of the existing GCPs, the remaining ones get all mixed up. For example, the linkages between the pixel locations and lat/long coords get switched for one or more control points.

Expected behavior Deleting any GCPs should not affect the other GCPs (obviously)

To Reproduce Steps to reproduce the behavior:

1. Find a layer and go to "edit georeferencing"
2. Delete a GCP somewhere in the middle of the list
3. Note, the warped preview should get weird
4. Select remaining GCPs in turn and some should clearly now be linked to incorrect locations.

Additional context This component could use a bit of a refactor, so fixing this bug should hopefully come along with some other code simplification.