Remove fallbacks related to container arguments

[erikbern]

This removes a bunch of code that should in theory be dead.

1. Assume we can always read the container arguments from the MODAL_CONTAINER_ARGUMENTS_PATH file
2. Assume that the app layout is always passed to the container as a part of that

We need fallback 1 for another couple of days just waiting for all workers to roll over.

Having fallback 2 has been incredibly useful over the last few weeks because we had to revert a bunch of server changes.

So I'm a bit nervous about merging this. Planning to let this PR sit for a few weeks and then merge it assuming things are good.

[erikbern]

@prbot approve

[modal-pr-review-automation]

Approved 👍. @mwaskom will follow-up review this.

[erikbern]

I think it's time to burn the bridges and ship this. Haven't run into any issues with the app layout for a few weeks now.

[mwaskom]

Sounds good.