(bug 45904) ReferencedEntitiesFinder::findSnakLinks handling of delete properties

Instead of looking at a Snak's property and that property's data type, ReferencedEntitiesFinder::
findSnakLinks will now only look at PropertyValueSnak's values and check whether they are of
the data value type 'wikibase-entityid'.
This handling makes more sense since a Snak's property and value could for some reason contain a
contradiction in which case the old handling would for example miss a reference by a
PropertyValueSnak's value because the Snak's property is deleted/missing (and therefore the type
can't be determined by the property) or because the property definition has changed while all
Snaks using that property have kept values suitable for the old property definition.

The new handling should also bring slightly improved performance since it does not require to
fetch the PropertyValueSnak's property to look up their type.

Change-Id: I3bb49eca1f20960a55cfdbd83599eb21d5d07903
Merge "(bug 45904) ReferencedEntitiesFinder::findSnakLinks handling of delete properties"