Remove the platform sections to avoid Xcode 12 issues.
Remove the platform sections to avoid Xcode 12 issues.

Take from https://github.com/google/gtm-session-fetcher/pull/202.

google/google-api-objectivec-client-for-rest,google/google-api-objectivec-client-for-rest,google/google-api-objectivec-client-for-rest,google/google-api-objectivec-client-for-rest,google/google-api-objectivec-client-for-rest