Network: Remove deprecated properties in SystemConfiguration

In change #52786 of phosphor-networkd, the deprecated DefaultGateway
and DefaultGateway6 properties of SystemConfiguration are removed.
Remove them in phosphor-dbus-interfaces accordingly.

Change-Id: Ic05baef397c3a3b4c8682fbe8de455358f1a4e53
Signed-off-by: Jiaqing Zhao <jiaqing.zhao@intel.com>
diff --git a/yaml/xyz/openbmc_project/Network/SystemConfiguration.interface.yaml b/yaml/xyz/openbmc_project/Network/SystemConfiguration.interface.yaml
index 0fc0ef7..a0a82c4 100644
--- a/yaml/xyz/openbmc_project/Network/SystemConfiguration.interface.yaml
+++ b/yaml/xyz/openbmc_project/Network/SystemConfiguration.interface.yaml
@@ -5,18 +5,3 @@
       type: string
       description: >
           The value of this property shall be host name of the system.
-    # Deprecating these "DefautGateway" and "DefaultGateway6" properties.
-    # These are no longer needed, as we are going to support Default gateway
-    # per Ethernet interface.
-    - name: DefaultGateway
-      type: string
-      description: >
-          default IPv4 gateway of the system.
-      errors:
-          - xyz.openbmc_project.Common.Error.InvalidArgument
-    - name: DefaultGateway6
-      type: string
-      description: >
-          default IPv6 gateway of the system.
-      errors:
-          - xyz.openbmc_project.Common.Error.InvalidArgument