Update secret detection template to remove fetch on historic scan

What does this MR do and why?

Fixes a bug described #357453 (closed) by removing the git fetch line for historic scans. git fetch --unshallow on complete repos (repos full cloned AFIAK) resulted in a fatal error that prevented the scan from running.

Describe in detail what your merge request does and why.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.