[Cleanup] Delete unused methods / imports / args / classes: 2.1/N

Created by: suchenzang

Splitting https://github.com/facebookresearch/metaseq/pull/231 up into 2 PRs.

Removed:

• unused moe_disable_padding arg
• unused from_pretrained methods, since we currently depend on load_model_ensemble_and_task from checkpoint_utils (not great but saving that for another PR)
• unused hub_models method