Remove DAST site profiles feature flag

• Related issue: #235531 (closed)

What does this MR do?

This removes the security_on_demand_scans_site_profiles_feature_flag feature flag.

We have verified that DAST site profiles can properly be associated with on-demand scans in %13.3, so it's now safe to remove the feature flag altogether.

How to test this?

• Open the On-demand scans form at /:group/:project/-/on_demand_scans.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry

As per the guidelines, this does not warrant a changelog entry because the feature flag being removed was already enabled by default.

A change that removes a feature flag should have a changelog entry - only if the feature flag did not default to true already.

• [-] 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.