cmd/keyserver-gcp: remove test user flags
cmd/keyserver-gcp: remove test user flags

There's no need for the test user flags in the GCP version of keyserver.
They should not have been included in the first place.

Change-Id: I4e16767ce4b3af730bc6d8bbba79b0cd9c08a503
Reviewed-on: https://upspin-review.googlesource.com/11520
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/gcp,upspin/gcp