[NDIS] NdisRegisterProtocol(): Add a comment about NDIS51 (reactos#8314)

CORE-20300

- `NdisRegisterProtocol()`: Add a comment about `NDIS51`: the
  `NDIS50_PROTOCOL_CHARACTERISTICS` structure is common to both
  NDIS 5.0 and 5.1.
  This is in contrast to the `NDIS50/NDIS51_MINIPORT_CHARACTERISTICS`
  structures, see for example commit 3e169e3 (r49961).

- Remove obsolete SOURCES file. Addendum to commit d9e83ed (r40142).

Author: SergeGautherie

drivers/network/ndis/ndis/SOURCES

@@ -1156,6 +1156,7 @@ NdisRegisterProtocol(
       break;
 
     case 0x05:
+      /* Identical for both NDIS50 and NDIS51 */
       MinSize = sizeof(NDIS50_PROTOCOL_CHARACTERISTICS);
       break;