chore: remove --config flag to load the bentoml runtime config

[jjmachan]

What does this PR address?

We used a --config flag in our bentoml cli to load the bentoml config. However, the recommended way to load the config is by setting the env var BENTOML_CONFIG to the config file's path. This PR will remove the --config flag from bentoml_cli

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[sauyon]

Wait, why are we doing this? This seems like a backwards-incompatible change.

[ssheng]

Wait, why are we doing this? This seems like a backwards-incompatible change.

Discussed with Sauyon offline.