Make the read ahead buffer unselectable

Here is the email loop talk about why it is better to remove the option:
https://groups.google.com/forum/#!topic/nuttx/AaNkS7oU6R0

Change-Id: Ib66c037752149ad4b2787ef447f966c77aa12aad
Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

graphics/vnc/server/Kconfig

@@ -7,7 +7,6 @@ menuconfig VNCSERVER
 	bool "VNC server"
 	default n
 	depends on NET_TCP && !NX_LCDDRIVER
-	select NET_TCP_READAHEAD
 	select NX_UPDATE
 	---help---
 		Enable support for a VNC Remote Frame Buffer (RFB) server.

graphics/vnc/server/vnc_server.h

@@ -60,10 +60,6 @@
 
 /* Configuration */
 
-#ifndef CONFIG_NET_TCP_READAHEAD
-#  error CONFIG_NET_TCP_READAHEAD must be set to use VNC
-#endif
-
 #ifndef CONFIG_NX_UPDATE
 #  error CONFIG_NX_UPDATE must be set to use VNC
 #endif