Remove obsolete DEFAULT_EMBEDDING_MODEL code

[simonw]

It's cheaper and better than ada-002.

llm/llm/cli.py

Line 45 in fcba89d

DEFAULT_EMBEDDING_MODEL = "ada-002"

UPDATE: I thought I needed to update this to 3-small but it turns out that code does nothing:

llm/docs/embeddings/cli.md

Line 32 in fcba89d

You can omit the `-m/--model` option if you set a {ref}`default embedding model <embeddings-cli-embed-models-default>`.