[Assistants] Fix `remove` behaviour (#869)
Follow up to https://github.com/huggingface/chat-ui/pull/859,
specifically to [this
comment](https://github.com/huggingface/chat-ui/pull/859#discussion_r1497978234)
Bug: when a user removes an assistant, the assistant was NOT being
removed from the settings list.
[This PR fixes this
problem](https://github.com/huggingface/chat-ui/pull/869#discussion_r1499470412)
