[AC-2522] Remove collection enhancements opt-in (#9283)

* Remove FlexibleCollectionsMigration feature flag and code

* Remove api method

libs/common/src/admin-console/abstractions/organization/organization-api.service.abstraction.ts

@@ -82,5 +82,4 @@ export class OrganizationApiServiceAbstraction {
     id: string,
     request: OrganizationCollectionManagementUpdateRequest,
   ) => Promise<OrganizationResponse>;
-  enableCollectionEnhancements: (id: string) => Promise<void>;
 }