Treat code-size warnings as errors

[frangio]

Follow up to #5099.

The current config is treating code size as a warning (true is equivalent to 'warn'), which would not cause CI to fail. This config removes the line so that it's considered an error (according to default: 'error' below).

Feel free to close this if it's not the desired behavior.

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[Amxx]

I'd say this is desirable, unless it triggers when compiling the "coverage" contract that are way bigger (because of the added events).

CI shows that it should be ok, so I'd be in favor of it. We can revert back if it turns out its causing unforseen issues.