veyron/runtimes/google/ipc: Revert <principal_pattern>@@<objectname>
veyron/runtimes/google/ipc: Revert <principal_pattern>@@<objectname>

This change (https://veyron-review.googlesource.com/#/c/3480/
and https://veyron-review.googlesource.com/#/c/3351/) breaks
cases where the server identifier is an endpoint.

git revert c68f1ffd7d79d68c9c5d68bd56b5687ef00d7ff1
git revert -m 1 c68f1ffd7d79d68c9c5d68bd56b5687ef00d7ff1

Change-Id: I5ea51f7db6217e57a1f91854f4257e5fab0c2721

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23