Massively refactor and sanify _extract_fixed_condition_columns

This work is based on d8b7d9f58..337f3ee80, which while taking the right
direction had too many loose ends. imlari++
The initial (now passing) test for RT#93244 (fixed in 9f171728)
Remove forgotten part of a48693f4 (incompletely reverted by e327f126)
Fix bogus reference to related class (introduced way back in 97c96475)