groups: Remove id_field_name field from GroupPermissionSetting.

id_field_name was being used only to set the initial dummy
values for realm settings. We can directly add "_id" to the
setting name instead of having an extra id_field_name field.

zerver/lib/types.py

@@ -314,7 +314,6 @@ class GroupPermissionSetting:
     allow_nobody_group: bool
     allow_everyone_group: bool
     default_group_name: str
-    id_field_name: str
     default_for_system_groups: str | None = None
     allowed_system_groups: list[str] = field(default_factory=list)