Un-type symbolic.flatten

This restores compatibility with pymbolic 2022.2

x-ref: firedrakeproject/loopy#27

loopy/symbolic.py

@@ -49,7 +49,6 @@
 import pymbolic.primitives as p
 import pytools.lex
 from islpy import dim_type
-from pymbolic import ArithmeticExpressionT
 from pymbolic.mapper import (
     CachedCombineMapper as CombineMapperBase,
     CachedIdentityMapper as IdentityMapperBase,
@@ -211,14 +210,8 @@ class FlattenMapper(FlattenMapperBase, IdentityMapperMixin):
     pass
 
 
-ArithmeticOrExpressionT = TypeVar(
-                "ArithmeticOrExpressionT",
-                ArithmeticExpressionT,
-                ExpressionT)
-
-
-def flatten(expr: ArithmeticOrExpressionT) -> ArithmeticOrExpressionT:
-    return cast(ArithmeticOrExpressionT, FlattenMapper()(expr))
+def flatten(expr):
+    return FlattenMapper()(expr)
 
 
 class IdentityMapper(IdentityMapperBase, IdentityMapperMixin):