Reverts https://github.com/zed-industries/zed/pull/44933.

It turns out that if you're copying agent responses to paste it anywhere
else that isn't the message editor (e.g., for a follow up prompt),
getting Markdown formatting is helpful. However, with the revert, the
underlying issue in https://github.com/zed-industries/zed/issues/42958
remains, so I'll reopen that issue, unfortunately.

Release Notes:

- N/A