gh-157217: Drop unused mappingproxy helper from internal header

Author: KingLizard1020

Include/internal/pycore_descrobject.h

@@ -24,21 +24,6 @@ extern PyTypeObject _PyMethodWrapper_Type;
 
 extern void *_PyMember_GetOffset(PyObject *, PyMemberDef *);
 
-/* Return a borrowed reference to the mapping wrapped by a mappingproxy.
- * The struct layout matches mappingproxyobject in Objects/descrobject.c.
- */
-static inline PyObject *
-_PyDictProxy_GetMapping(PyObject *op)
-{
-    typedef struct {
-        PyObject_HEAD
-        PyObject *mapping;
-    } _PyMappingProxyObject;
-    assert(op != NULL);
-    assert(PyObject_TypeCheck(op, &PyDictProxy_Type));
-    return ((_PyMappingProxyObject *)op)->mapping;
-}
-
 #ifdef __cplusplus
 }
 #endif