tests(abtesting): remove cocoapods CI

[daymxn]

Per b/561540253,

This PR is part of the incremental work to remove CocoaPods infrastructure usage from our testing workflows in CI, as CocoaPods will be going away soon.

More specifically, this PR removes the CocoaPods workflow run that was invoked in the FirebaseABTesting workflow. The existing _spm.yaml workflow that was invoked with ABTestingUnit already covers the same use-cases (including catalyst), so no additional changes are needed.

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[ncooke3]

I believe we should keep CocoaPods CI around. It offers test coverage that we don't yet have with SPM.