debugfs_known_mountpoints[] is not used any more, so let's remove it.

Link: http://lkml.kernel.org/r/1535102651-19418-1-git-send-email-n-horiguchi@ah.jp.nec.comSigned-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>