Refactor: Remove unused weakref import and class_registry

Author: stickm4n

sqlmodel/main.py

@@ -6,7 +6,6 @@
 import types
 import typing
 import uuid
-import weakref
 from collections.abc import Callable, Mapping, Sequence, Set
 from dataclasses import dataclass
 from datetime import date, datetime, time, timedelta
@@ -846,8 +845,6 @@ def resolve_type_alias(annotation: Any) -> Any:
     return resolution
 
 
-class_registry = weakref.WeakValueDictionary()
-
 default_registry = registry()
 
 _TSQLModel = TypeVar("_TSQLModel", bound="SQLModel")