Delete duplicate item in __all__ (#1993)

There are two `SmallIntField` in `__all__` of file `fields.__init__.py`.

Author: seamile

tortoise/fields/__init__.py

@@ -62,7 +62,6 @@
     "IntField",
     "JSONField",
     "SmallIntField",
-    "SmallIntField",
     "TextField",
     "TimeDeltaField",
     "UUIDField",