refactor(firebase_core, web): remove variant fallback in registerVersion #2112

Job	Run time
web (packages/cloud_firestore/cloud_firestore/example)	13m 15s
web-wasm (packages/cloud_firestore/cloud_firestore/example)	15m 8s
web-app-check	6m 58s
web (tests)	15m 12s
web-wasm (tests)	7m 55s
web (tests)	11m 44s
web-wasm (packages/cloud_firestore/cloud_firestore/example)	14m 38s
web (packages/cloud_firestore/cloud_firestore/example)	13m 15s
web-app-check	6m 58s
web-wasm (tests)	7m 55s
	1h 52m 58s

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