Remove old relations (#11993)

This is a first PR to remove old relation logic

Next steps:
- remove relationMetadata from cache
- remove relationMetadata table content and structure
- refactor relationDefinition to leverage field.settings instead

packages/twenty-server/src/engine/core-modules/feature-flag/enums/feature-flag-key.enum.ts

@@ -7,6 +7,5 @@ export enum FeatureFlagKey {
   IsUniqueIndexesEnabled = 'IS_UNIQUE_INDEXES_ENABLED',
   IsJsonFilterEnabled = 'IS_JSON_FILTER_ENABLED',
   IsCustomDomainEnabled = 'IS_CUSTOM_DOMAIN_ENABLED',
-  IsNewRelationEnabled = 'IS_NEW_RELATION_ENABLED',
   IsPermissionsV2Enabled = 'IS_PERMISSIONS_V2_ENABLED',
 }