Prohibit customize export config (#337)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/337

More cleanup in `executorch/examples`.

`_unlift` flag is removed for all models and all use-cases in examples, we can push to use canonical export config one step further.

Not in this diff but `enable_aot` will be removed later together with `exir.capture()`

Reviewed By: digantdesai

Differential Revision: D49258936

fbshipit-source-id: 09411f4f6df6c09b7a91dad2de2c93fda6e12214