chore(auth-mailchimp-sync) remove auth-mailchimp-sync extension

[barticus]

The auth-mailchimp-sync extension was migrated to https://github.com/mailchimp/Firebase a number of years ago, and that should be referred to for the latest version of the extension and for feature requests or bug fixes.

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[barticus]

I have signed the CLA, please retrigger the action for it to be verified.

[pr-Mais]

Apologies, I didn't notice this PR and removed the extension in another PR, closing since it's resolved now, thanks!