remove duplicate deepcopy (#2550)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2550

Program is deep copied inside backend_api before being passed to preprocess:
https://www.internalfb.com/code/fbsource/fbcode/executorch/exir/backend/backend_api.py?lines=106

And then deepcopied again here.

Reviewed By: cccclai

Differential Revision: D55133796

fbshipit-source-id: ce7df2803d24cb2de1e64c0067b1ff71681ded9e