Delete SAST Rule Enhancement.md

We never needed a template that was stored in GitLab's Git repo.

Indeed, we used ICBM to bulk create issues, which uses templates stored locally on disk.

Thus, it makes sense to store those templates in repos the VR team is DRI for (eg. sast-rules), and not here where every single change requires external approval for no benefit.

Lastly, work on Add or overhaul SAST rules to cover additional ... (&10971 - closed) is now complete.