grpcauth: silence annoying log messages
grpcauth: silence annoying log messages

All tests pass.

Change-Id: I93992126945dfdbd242de090519765f45ebbc28d
Reviewed-on: https://upspin-review.git.corp.google.com/2107
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

upspin/upspin,upspin/upspin