Remove unused atom14 default-frame gather

[taivu1998]

Summary

• Remove the unused default_frames[aatype, ...] gather from frames_and_literature_positions_to_atom14_pos.
• Keep the rigid-group count explicit by deriving num_rigid_groups from default_frames.shape[-3] before building the atom-to-frame one-hot mask.
• Add a regression test with a shape-only default_frames sentinel so the helper fails if it tries to gather default frames again.

Root cause

Issue #249 points out that frames_and_literature_positions_to_atom14_pos gathered default_frames by residue type even though the function selects per-atom rigid groups from group_idx and lit_positions. The gathered tensor was unused, and the surrounding comment suggested an atom14-shaped tensor even though default_frames is organized by the 8 rigid groups. Removing the dead gather keeps the implementation aligned with the actual atom14-to-rigid-group mapping and avoids a confusing, unnecessary tensor operation.

Validation

• python -m py_compile openfold/utils/feats.py tests/test_feats.py
• git diff --check
• Direct regression check with a default_frames sentinel whose getitem raises, confirming the helper only needs the shape and still returns (1, 3, 14, 3)
• Attempted focused pytest with python -m pytest tests/test_feats.py -k frames_and_literature_positions_to_atom14_pos -q; local collection is blocked by missing environment dependencies (ml_collections, and lightweight direct imports also showed missing tree/Bio outside the full repo environment)

Fixes #249