Remove stray enum instance

[goto-bus-stop]

This enum LOG_LEVELS declaration also included a variable name, causing https://github.com/RuntimeTools/appmetrics/issues/641. It ended up defining a Log_levels variable in every compilation unit that included "Log.h", which then caused duplicate symbols at link time.

I don't know if there was a reason for this definition, but it looks like a bug to me … if not, maybe it can be fixed by just putting a static in front? idk.

Thanks!