ksm: remove redundant declarations in ksm.h

ksm: remove redundant declarations in ksm.h

Currently, for struct stable_node, no one uses it in both the
include/linux/ksm.h file and the file that contains it.  For struct
mem_cgroup, it's also not used in ksm.h.  So they're all redundant, just
remove them.

Link: https://lkml.kernel.org/r/20220831031951.43152-4-zhengqi.arch@bytedance.com
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>