Fix permission flag deletion validator (#19636)

As title

packages/twenty-server/src/engine/workspace-manager/workspace-migration/workspace-migration-builder/validators/services/flat-permission-flag-validator.service.ts

@@ -229,13 +229,7 @@ export class FlatPermissionFlagValidatorService {
         flatEntityMaps: flatRoleMaps,
       });
 
-      if (!isDefined(referencedRole)) {
-        validationResult.errors.push({
-          code: PermissionsExceptionCode.ROLE_NOT_FOUND,
-          message: t`Role not found`,
-          userFriendlyMessage: msg`Role not found`,
-        });
-      } else if (!referencedRole.isEditable) {
+      if (isDefined(referencedRole) && !referencedRole.isEditable) {
         validationResult.errors.push({
           code: PermissionsExceptionCode.ROLE_NOT_EDITABLE,
           message: t`Role is not editable`,