clean up the PR job script for iOS build (#26353)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26353
### Summary
As the new iOS building script has been landed, this PR will clean up some redundant code for the PR jobs.
### Test Plan
- Don't break any existing CI jobs
- Don't break the old iOS CI jobs
Test Plan: Imported from OSS
Differential Revision: D17457253
Pulled By: xta0
fbshipit-source-id: 0d85117533a62d0b9b7b859b0044fd4388c3c9d4
