Spatial index builder should use linked_location_id sample attribute data

[johnvanbreda]

To reproduce:

1. Create a sample attribute with the system function set to linked_location_id (integer).
2. Add a control to a recording form so the user can fill in this value, e.g.:

   [smpAttr:48]
   @control_type=location_autocomplete
   @autofillFromLocationTypeId=15
   @extraParams={"location_type_id":15}

3. Add some records which straddle the boundary of the locations in the layer (in this example 15 is the vice county layer). You are asked to pick which side of the boundary the record lies.
4. Now save the records.
5. Once the records are spatially indexed, example cache_samples_functional.location_ids and you will see that both location IDs have been indexed for each side of the boundary. The expected outcome is that just the one you chose is indexed.

[johnvanbreda]

Fixed in the develop branch.