Retire the use of grouping property for grouping predicate

Following the limitation that only properties were supported
as grouping concept, the line objects were taking a property
as parameter and adding a hardcoded wdt: prefix.

Now that we support any type of predicate for grouping,
let's have the line objects use that predicate directly.

The GroupingConfiguration objects now have a get_predicate method,
and that's where the wdt: is added for the PropertyGroupingConfiguration.

Also solves a hard crash [1] when using the queries feature
on dashboards using Predicates as grouping.

[1] Traceback

AttributeError: 'PredicateGroupingConfiguration' object has no attribute 'property'

Bug: T312729