Revise `DeclaredQuery` count query derivation

[mp911de]

DeclaredQuery.deriveCountQuery(@Nullable String countQuery, @Nullable String countQueryProjection) is a strange way encapsulating a conditional block. Also, if the DeclaredQuery is already a count query: What is that method supposed to do?

[mp911de]

Superseded by #3527 and #2989 that also require a type split.