Remove warning log messages for Long Running Operations

[derekbekoe]

No description provided.

[azurecla]

Hi @derekbekoe, I'm your friendly neighborhood Azure Pull Request Bot (You can call me AZPRBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no contribution license agreement is required at this point. Real humans will now evaluate your PR.

TTYL, AZPRBOT;

[yugangw-msft]

i like this change personally, but for the people get used to interactive flow, is it possible we can still output the starting message, but just don't prefix with "warning:"?

[derekbekoe]

'WARNING:' is not displayed in interactive mode (if there's a TTY available).
But log message are prefixed with the log level if there is no TTY (e.g. redirecting the output to file) as was the case in #572.

[tjprescott]