The abort happened when a query contained a conjunctive predicate
of the form 'view column = constant' in the WHERE condition and 
the grouping list also contained a reference to a view column yet
a different one.

Removed the failing assertion as invalid in a general case.

Also fixed a bug that prevented applying some optimization for grouping
queries using views. If the WHERE condition of such a query contains
a conjunctive condition of the form 'view column = constant' and
this view column is used in the grouping list then grouping by this
column can be eliminated. The bug blocked performing this elimination.