Add option to replace nested value by deleting it(prev nested obj)

[yezyilomo]

Add option to delete previous value(Obj) of a nested field if the new value is None. i.e

If obj.field == obj1

Then we do obj.field = None

Then what do we do to the obj1?

Currently it remains unaltered, but there’re scenarios where if you replace it with None you won’t need it anymore, so in this case it’s better to delete it immediately after replacing it with None.

So we'll have the arg like "delete_on_replacing" on NestedField for enabling this behavior.