remove selector from tutorial

Summary:
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