Revert "compat: skb_mark_not_on_list will be backported to Ubuntu 18.04"

This reverts commit cad8059.

Because this commit has not been backported so far, due to the implications
of building Ubuntu's backport of wireguard in a timely manner.

For now, reverting this fix would allow wireguard-linux-compat CI to work
on Ubuntu 18.04.

A different fix or the same one can be applied again when the time is
right.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Author: Thadeu Lima de Souza Cascardo

src/compat/compat.h

@@ -823,7 +823,7 @@ static __always_inline void old_rcu_barrier(void)
 #define COMPAT_CANNOT_DEPRECIATE_BH_RCU
 #endif
 
-#if (LINUX_VERSION_CODE < KERNEL_VERSION(4, 19, 10) && LINUX_VERSION_CODE >= KERNEL_VERSION(4, 15, 0) && !defined(ISRHEL8) && !defined(ISUBUNTU1804)) || LINUX_VERSION_CODE < KERNEL_VERSION(4, 14, 217)
+#if (LINUX_VERSION_CODE < KERNEL_VERSION(4, 19, 10) && LINUX_VERSION_CODE >= KERNEL_VERSION(4, 15, 0) && !defined(ISRHEL8)) || LINUX_VERSION_CODE < KERNEL_VERSION(4, 14, 217)
 static inline void skb_mark_not_on_list(struct sk_buff *skb)
 {
 	skb->next = NULL;