NetworkPkg/Dhcp6Dxe: Fix extraneous parentheses

Without this change we get:

  error: equality comparison with extraneous parentheses

when building with -D NETWORK_IP6_ENABLE on XCODE5.

Signed-off-by: Mike Beaton <mjsbeaton@gmail.com>

NetworkPkg/Dhcp6Dxe/Dhcp6Utility.c

@@ -1001,7 +1001,7 @@ Dhcp6AppendETOption (
     return EFI_INVALID_PARAMETER;
   }
 
-  if ((Elapsed == NULL)) {
+  if (Elapsed == NULL) {
     return EFI_INVALID_PARAMETER;
   }