Firebase Messaging - Fix TypeScript types

[gauntface]

Fix up Typescript types so that everything can be auto-generated from the Typescript src rather than a separate typescript types file.

[google-oss-bot]

Hey there! I couldn't figure out what this issue is about, so I've labeled it for a human to triage. Hang tight.

[google-oss-bot]

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[mmermerkaya]

I'm closing this issue as Messaging's TS types are fixed and there's no need for a separate file anymore.