remove RT_LWIP_TCP_SND_BUF setting

When not configured in rtconfig.h, lwipopt.h will give us a reasonable
value(TCP_MSS * 2).

bsp/sam7x/rtconfig.h

@@ -113,9 +113,6 @@
 /* the number of simulatenously active TCP connections*/
 #define RT_LWIP_TCP_PCB_NUM	5
 
-/* TCP sender buffer space*/
-#define RT_LWIP_TCP_SND_BUF	1500
-
 /* Enable SNMP protocol*/
 /* #define RT_LWIP_SNMP */