Remove .cursorrules in favor of AGENTS.md (#3559)

Remove the .cursorrules symlink as [Cursor now supports
AGENTS.md](https://docs.cursor.com/en/context/rules) directly for
defining agent behavior.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: lennartkats-db