Clean up and remove duplicate code in Travis iOS/tvOS test scripts

Summary:
**Motivation**

Remove duplicated lines in objc-test-ios.sh and objc-test-tvos.sh

**Test plan**

Travis CI should continue to work as before.
Closes https://github.com/facebook/react-native/pull/13173

Differential Revision: D4812177

Pulled By: hramos

fbshipit-source-id: 40afe0beab896d741f607f4841df7e9b42f4c17d