Remove unused L2TPIPsec.GroupName property

kL2tpIpsecGroupNameProperty is not used in shill or chrome,
and I've confused it at least once with kL2tpIpsecTunnelGroupProperty.
Make it go away.

BUG=chromium:224834
TEST=Recompile shill, codesearch in cs.chromium.org

Change-Id: I5d288043c8fa0d83b60f75a230cf1d11cfb163e2
Reviewed-on: https://gerrit.chromium.org/gerrit/49909
Reviewed-by: Philipp Neubeck <pneubeck@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
diff --git a/dbus/service_constants.h b/dbus/service_constants.h
index b61f396..e9a9705 100644
--- a/dbus/service_constants.h
+++ b/dbus/service_constants.h
@@ -595,7 +595,6 @@
 const char kL2tpIpsecPskRequiredProperty[] = "L2TPIPsec.PSKRequired";
 const char kL2tpIpsecUserProperty[] = "L2TPIPsec.User";
 const char kL2tpIpsecPasswordProperty[] = "L2TPIPsec.Password";
-const char kL2tpIpsecGroupNameProperty[] = "L2TPIPsec.GroupName";
 // deprecated:
 const char kL2tpIpsecClientCertSlotProp[] = "L2TPIPsec.ClientCertSlot";