[Cleanup]: remove upgrade_state_dict logic, normformer, unused alignment/encoder args

Created by: suchenzang

Tackling https://github.com/facebookresearch/metaseq/issues/165

Only checkpoints to work with here are with OPT models so far.

More cleanup around unused args / logic in preparation for reducing complexity around configuration / checkpointing logic.

• Removed all normformer args & related classes (sync ln)
• Removed stable embeddings
• Removed FeedForwardNetwork class