Revert D24165889: Update native_functions.yaml to add DefaultBackend.
Test Plan: revert-hammer
Differential Revision:
D24165889 (https://github.com/pytorch/pytorch/commit/1f9ddf64d21a24b30a947ec2d50cc99cdba8b626)
Original commit changeset: 7f3ccdb3499b
fbshipit-source-id: b5d0de57d918011f1e19c9ef6aafa89fefcb42d5
