mm: memcontrol: drop dead CONFIG_MEMCG_SWAP config symbol

Since 2d1c498072 ("mm: memcontrol: make swap tracking an integral part
of memory control"), CONFIG_MEMCG_SWAP hasn't been a user-visible config
option anymore, it just means CONFIG_MEMCG && CONFIG_SWAP.

Update the sites accordingly and drop the symbol.

[ While touching the docs, remove two references to CONFIG_MEMCG_KMEM,
  which hasn't been a user-visible symbol for over half a decade. ]

Link: https://lkml.kernel.org/r/20220926135704.400818-5-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeelb@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/swap_cgroup.h

@@ -4,7 +4,7 @@
 
 #include <linux/swap.h>
 
-#ifdef CONFIG_MEMCG_SWAP
+#if defined(CONFIG_MEMCG) && defined(CONFIG_SWAP)
 
 extern unsigned short swap_cgroup_cmpxchg(swp_entry_t ent,
 					unsigned short old, unsigned short new);
@@ -40,6 +40,6 @@ static inline void swap_cgroup_swapoff(int type)
 	return;
 }
 
-#endif /* CONFIG_MEMCG_SWAP */
+#endif
 
 #endif /* __LINUX_SWAP_CGROUP_H */