remove selector from tutorial
This is motivated by a conversation I had with a confused user.
I don't mean to argue that we shouldn't have selector in the tutorial in general. But I think the way we introduce it causes confusion:
- It's a tricky concept, and we introduce it in the same breath we're introducing a different tricky concept (dagster_type_loaders).
- It's not actually necessary in the place where we introduce it. There's only one format to choose from, and there's no sub-config that depends on that format.
Test Plan: bk
Reviewers: max, sashank, yichendai
Reviewed By: max
Differential Revision: https://dagster.phacility.com/D4763