Remove security_on_demand_scans_feature_flag

What does this MR do?

this merge request removes all traces of security_on_demand_scans_feature_flag. it was previously set to enabled by default and is safe to remove.

Related Issue(s)

• #240879 (closed)
• #237796 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry feature flag was already enabled by default
• 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