ar71xx: remove obsolete '< KERNEL_VERSION(4, 2, 0)' check

Because the oldest supported kernel version on the ar71xx target is 4.4,
the condition that the kernel version is smaller than 4.2.0 is always
false. Remove the obsolete check from ag71xx_main.c to clean up the code
a bit.

Signed-off-by: Gabor Juhos <juhosg@freemail.hu>

target/linux/ar71xx/files/drivers/net/ethernet/atheros/ag71xx/ag71xx_main.c

@@ -13,13 +13,6 @@
 
 #include "ag71xx.h"
 
-#if LINUX_VERSION_CODE < KERNEL_VERSION(4,2,0)
-static inline void skb_free_frag(void *data)
-{
-	put_page(virt_to_head_page(data));
-}
-#endif
-
 #define AG71XX_DEFAULT_MSG_ENABLE	\
 	(NETIF_MSG_DRV			\
 	| NETIF_MSG_PROBE		\