Drop restore_checkpoint_fusion_state
Nothing sets `_native_fused_projections`, so the method was a no-op on every real
checkpoint despite promising to restore the checkpoint's fusion state. Wiring it
up needs fused-checkpoint loading first, which no fusable module supports -- they
only ever get `to_qkv` from `fuse_projections`, so loading fused weights drops
them as unexpected keys. Removed rather than left as a stub; it can come back
with the loading support it needs.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
