Fix delete_methods

Hi~ is there any need to `del self.delete_methods` here?
And actually `del self.delete_methods[:]` seems not do anything ,instead of copy the original list and delele the
reference . 

I guess that you just want deletion_methods work on  one of `self.weak_ref` and `self.weak_func` but not both.
It should be ok if using  `del self.delete_methods`