rpc: remove log print that caused a panic
rpc: remove log print that caused a panic

If a user without a Factotum tries to make an authenticated RPC request
then this error path will be taken, but then trying to log the
Factotum().PublicKey() would trigger a panic. Remove the redundant log
message; the returned error should be sufficient.

Change-Id: I6c98637b753773d3805a06c763700505e9a8e04b
Reviewed-on: https://upspin-review.googlesource.com/13320
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin