Remove an unneeded variable assignment.

Found using Clang's static analyzer.

Objects/tupleobject.c

@@ -86,7 +86,6 @@ PyTuple_New(register Py_ssize_t size)
 		{
 			return PyErr_NoMemory();
 		}
-		nbytes += sizeof(PyTupleObject) - sizeof(PyObject *);
 
 		op = PyObject_GC_NewVar(PyTupleObject, &PyTuple_Type, size);
 		if (op == NULL)