fix(serverless): clean up ID token example

[ace-n]

NOTE: Since this was a pretty minor change, I did not test this on GCF or Cloud Run.

Please LMK if I should change that!

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/idtokens-serverless.js:41, tag functions_bearer_token
• samples/idtokens-serverless.js:44, tag cloudrun_service_to_service_auth

You are about to delete 2 region tags.

• samples/idtokens-serverless.js:31, tag google_auth_idtoken_serverless
• samples/idtokens-serverless.js:33, tag run_service_to_service_auth

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[ace-n]

(The deleted region tags are not used in the docs.)