QRemoteObjectRegistry: fix the property initialization

For some reason, QRemoteObjectRegistry initialized its property storage
with three elements. However, both this class and the matching
QRemoteObjectRegistrySource only have one property - sourceLocations.
Also, nothing in the implementation ever accesses the properties with
indices 1 and 2.

Remove the two unneeded properties from the initialization.
This fix is required for a follow-up patch that introduces a strict
property count check between the source and the replica.

Amends ec68d2724872c4e3c0b63e0ac12a369e639088de (the first commit in
the module).

Task-number: QTBUG-150780
Pick-to: 6.12 6.8 6.5 5.15
Change-Id: I8dbe9e72ffd3cd6c5246e0c1016b2360679c780e
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>