Merging 'where' and polymorphic login in filter_subtable

I decided to go for simplicity and less code to test over
convinience and syntactic sugar. Basically, the new where
syntax that I introduced superceeds the old polymorphic
syntax with "type" and "type_key". I decided that there
is no reason to keep both, and thus deleted bunch of code.

Existing Shrinkfiles will have to be rewritten, as this is
incompatible change.