nit: Remove dangling string (apache#2650)

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Author: Fokko

pyiceberg/expressions/__init__.py

@@ -373,8 +373,6 @@ def __getnewargs__(self) -> Tuple[BooleanExpression]:
         """Pickle the Not class."""
         return (self.child,)
 
-    """TRUE expression."""
-
 
 class AlwaysTrue(BooleanExpression, Singleton, IcebergRootModel[str]):
     """TRUE expression."""