Cleanup `cmd/root/auth.go`

[nfx]

TODO:

• improve profile selection
• don't change the auth behavior
• try improving UI prompt for profile selectors - possibly with introducing alternative cmdio.Select.

Follow-up to:

This is counter intuitive. I specify e2-dogfood, happen to have an expired token, and when running a command with --profile e2-dogfood am prompted to select a profile. I would rather see an error saying the token has expired.

Originally posted by @pietern in #357 (comment)

[andrewnester]

Has been addressed as part of this PR #502