Remove a broken link to prompting_guide.md in docs/getting-started.md (#2858)

The file no longer exists. We've been receiving this feedback several
times.
- https://github.com/openai/codex/issues/2374
- https://github.com/openai/codex/issues/2810
- https://github.com/openai/codex/issues/2826

My previous PR https://github.com/openai/codex/pull/2413 for this issue
restored the file but now it's compatible with the current file
structure. Thus, let's simply delete the link.