Chat/Commands: Remove LLM reranker for repos without embeddings

[umpox]

Part of #1544
Part of #1464

Description

The LLM reranker adds a significant amount of latency to reranking any fetched context. This applies to all commands, chat and edits. The latency is typically between 2-5 seconds.

This PR just disables the reranker for now.

I think this is worth doing as:

• The added latency from the reranker is not worth the possible benefit right now. It still can sometimes rerank poorly and doesn't provide enough value to justify waiting that long.
• Context doesn't have a constant value, in many cases context will provide little value (e.g. simple fixups) in some cases, like chat, context would provide a lot more value. We don't have a way of determining that right now, so we're often adding lots of latency for no good reason.

I created an issue here: #1721 to evaluate a different or improved approach. Possible details there on how we could improve this.

Test plan

On a repository without embeddings, run:

• Chat
• Commands
• Edits

Review results and context included.

[abeatrix]

Thank you!