Don't send dummy App Check token to functions endpoint

[hsubox76]

See firebase/flutterfire#6794

This is a change being made across Android, iOS, and Web.

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Latest commit: c5212c2

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[github-actions]

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[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (1b33fda) and head commit (85b17f6).

Test Logs

• Base (1b33fda): https://github.com/firebase/firebase-js-sdk/actions/runs/1183629531
• Head (85b17f6): https://github.com/firebase/firebase-js-sdk/actions/runs/1184162142