Spec: remove multi_phy_tx spec text and capability bit

This flag was added on March 3, 2008 to comply with a specific vendor's
hardware limitations.  This vendor is out of business, and with the
assumption of a software datapath backup, it is unlikely that a switch
would be unable to support sending to a physical port and the controller
simultaneously.

This commmit removes the corresponding spec text and replaces the
capability bit with a reserved one.

Thanks to Justin Pettit for the history.

udatapath/datapath.c

@@ -73,8 +73,7 @@ extern char serial_num;
 #define OFP_SUPPORTED_CAPABILITIES ( OFPC_FLOW_STATS        \
                                      | OFPC_TABLE_STATS        \
                                      | OFPC_PORT_STATS        \
-                                     | OFPC_QUEUE_STATS        \
-                                     | OFPC_MULTI_PHY_TX )
+                                     | OFPC_QUEUE_STATS )
 
 /* Actions supported by this implementation. */
 #define OFP_SUPPORTED_ACTIONS ( (1 << OFPAT_OUTPUT)         \