Historically, all log.Err and log.Errf logs have been automatically given the [AUDIT] tag, just in case the error is significant enough that we want to track it. However, this runs counter to our goals of having regimented, structured, useful audit logs. So make Err and Errf behave exactly like their Warning/Info/Debug brethren, and not automatically add the [AUDIT] tag.

I examined all uses of these methods via a regex search for [Ll]og.*\.Errf?\(. In each instance I made a judgement call of whether the usage a) should be upgraded to AuditErrf to retain the [AUDIT] tag, b) should be left as-is so it will be logged as a non-audit error, or c) something else entirely, like downgrading to a Warning-level log.

As a result, all uses of the non-formatting log.Err function were removed, so I also removed it from the Logger interface.

Part of #8554