shill: vpn: OpenVPN doesn't register a device on an RPC path.

... so getting the device RPC ID for a VPN service is not supported.

BUG=chromium-os:26989
TEST=ran on device

Change-Id: Ief8f7aa1c633f1459c2472bcae0ed3311657076c
Reviewed-on: https://gerrit.chromium.org/gerrit/20456
Commit-Ready: Darin Petkov <petkov@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Darin Petkov <petkov@chromium.org>
diff --git a/vpn_service.cc b/vpn_service.cc
index db85709..7573613 100644
--- a/vpn_service.cc
+++ b/vpn_service.cc
@@ -81,7 +81,6 @@
 }
 
 string VPNService::GetDeviceRpcId(Error *error) {
-  NOTIMPLEMENTED();
   error->Populate(Error::kNotSupported);
   return "/";
 }