Revise azidentity logging

[chlowell]

A couple small changes I hope will make authentication logs less noisy and more useful:

• Stop logging errors returned from GetToken() because the application gets the error and is compelled to handle it
• Remove "Azure Identity => " prefix because listeners don't need it for filtering or formatting

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[richardpark-msft]

Looks good, and it's a good point. I was thinking about this the other day so this is pretty timely :)