Remove existing location options on the add localization form location select element

Problem/Motivation

When adding new localization variants, it is possible to add a localization variant for a location that has an existing localization variant, but this should not be possible.

Steps to reproduce

1. Create a content type `/admin/structure/types/add` or use an existing content type.
2. Ensure content entities are enabled for localization `admin/config/variants`.
3. Add a node `/node/add`.
4. Add a localization for the node `/node/{node id}/location/add/{localization id}`.
5. Add another localization and note that the location you selected for the localization you previously created is present in the "Location" select list.

Proposed resolution

Check for any existing localization variant entities related to the current node, and remove their locations from the "Location" select list on the "Add Localization" form page.

Remaining tasks

• Review patch.