Revert "ipv4: tunnel: fix compilation on ARCH=um"

This reverts commit 06a7a37b.

The bug was already fixed, this added a dup include.
Reported-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel_core.c

@@ -38,7 +38,6 @@
 #include <net/geneve.h>
 #include <net/vxlan.h>
 #include <net/erspan.h>
-#include <net/ip6_checksum.h>
 
 const struct ip_tunnel_encap_ops __rcu *
 		iptun_encaps[MAX_IPTUN_ENCAP_OPS] __read_mostly;