Remove dead code: experimental pipelines, unused vars, unused fixture

Delete 6 validated-negative pipeline files (universal, soft_vocab,
rosetta_seq) and strip their branches from benchmark runners (~2000
lines). Remove unused _CONNECTOR_NAMES set from __init__.py and
unused model_config_dict_tied fixture from conftest.py. 403 tests pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: SStas