gpui: use a separate image in the image example and remove unnecessary examples (#8181)

Follow-up of https://github.com/zed-industries/zed/pull/8174#issuecomment-1959031659,
Fixes image example and removes window-less "noop" example.

<img width="1975" alt="Screenshot 2024-02-22 at 17 34 15"
src="https://github.com/zed-industries/zed/assets/9134003/060d8484-63b6-415a-9f06-189542422457">

Release Notes:
- N/A