[mypyc] Fix incorrect borrowed flag for setdefault() primitive (#12514)

`CPyDict_SetDefaultWithNone` doesn't return a borrowed reference,
since it just calls `CPyDict_SetDefault` which doesn't return a
borrowed reference.

Author: JukkaL

mypyc/primitives/dict_ops.py

@@ -140,7 +140,6 @@
     arg_types=[dict_rprimitive, object_rprimitive],
     return_type=object_rprimitive,
     c_function_name='CPyDict_SetDefaultWithNone',
-    is_borrowed=True,
     error_kind=ERR_MAGIC)
 
 # dict.setdefault(key, empty tuple/list/set)