remove old architecture pipeline for RNTester (#42245)

Summary:
As discussed with cipolleschi, RNTester shouldn't be tested for Old Arch. This PR removes those unnecessary pipeline runs

## Changelog:

[INTERNAL] [REMOVED] - remove old architecture pipeline for RNTester

Pull Request resolved: https://github.com/facebook/react-native/pull/42245

Test Plan: CI Green

Reviewed By: cortinico

Differential Revision: D52694176

Pulled By: cipolleschi

fbshipit-source-id: a607bac4659b0611d5f49b5e45134f896bb96a91