Remove Security MR pipeline config example because it breaks product functionality

What does this MR do and why?

Removes an example from our documentation about using Security scanners with MR pipelines that is known to break functionality.

Why?

Support receives tickets from Ultimate customers who've implemented what we have in our docs saying that it doesn't work as expected and it breaks the ability to view/inteact with vulnerabilities in the UI.

Examples

• https://gitlab.com/gitlab-gold/security-scan-mr-demo has been set up to show how adding rules: - if: '$CI_PIPELINE_SOURCE == "merge_request_event"' will break things and cause unexpected behavior
• https://gitlab.com/gitlab-gold/security-scan-mr-demo2 has been set up to demonstrate the expected behavior of running SAST and Dependency Scanning jobs on an Ultimate license

Feature	Default Result (w/ CI_COMMIT_BRANCH)	Result (w/ CI_PIPELINE_SOURCE == MR)
Vulnerability Report	Vulnerabilities detected on project default branch show up in Vulnerability Report	No vulnerabilities detected, Vulnerability Report is empty
Security Dashboard	Vulnerabilities detected on default used to populate Security Dashboard	Security Dashboard is empty
Merge Request Widget	Only vulnerabilities not present on project's default branch are show in MR widget	Every vulnerability detected in project's default branch shows up in MR widget
Security Pipeline Tab	Only vulnerabilities not present on project's default branch are show in pipeline security tab	Every vulnerability detected in project's default branch shows up in Pipeline security tab
Security scanners only run if supported languages or frameworks files are detected in repo	Security scanners only run when supported programming languages and frameworks are detected in codebase	Broken - All Security Scans run on all Merge Requests even if there are zero files of that type to scan
SAST_DISABLED / DEPENDENCY_SCANNING_DISABLED job variables	Respected	Ignored - all 18 SAST/DS scanner jobs ran even though SAST and DS were explicitly disabled
SAST/DS EXCLUDED_ANALYZERS job variables	Respected	Ignored - all 18 SAST/DS scanner jobs ran even though SAST and DS were explicitly excluded

Additional context and examples in this issue thread: #351937 (comment 991360826)

How to set up and validate locally

• Import https://gitlab.com/gitlab-gold/security-scan-mr-demo and https://gitlab.com/gitlab-gold/security-scan-mr-demo2 into different projects.
• Make a minor merge request (simple change to README.md) on both
• Observe each of the examples of the problem outlined above create problems when using - if: $CI_PIPELINE_SOURCE == "merge_request_event" rules in security-scan-mr-demo while none of these problems exist with security-scan-mr-demo2 (which uses defaults)

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.

• Related to #351937 (closed)
• Related to #369947 (closed)