Remove Learning to Rank code

[sihugh]

We disabled Learning to Rank earlier.

The PR removes the code that trains, deploys and invokes Learning to Rank.

I've added an ADR listing some of the reasons.

There are other things that need to be done, such as:

• removing the cronjob and env vars for LTR
• removing the infrastructure that supports our use of SageMaker
• references to LTR in finder-frontend