Remove `store_security_findings` feature flag related code

What does this MR do?

This MR removes the store_security_findings feature flag which has been introduced in %13.6.

The feature behind the flag is implemented as an experiment to improve the performance of the "pipeline security tab" for #267021 (closed). You can check the following comments to see how this feature improved the performance; #267021 (comment 451232588), #267021 (comment 451645316).

Related to #276011 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team