f2fs: drop duplicate header timer.h

Drop duplicate header timer.h from segment.c.
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -16,7 +16,6 @@
 #include <linux/kthread.h>
 #include <linux/swap.h>
 #include <linux/timer.h>
-#include <linux/timer.h>
 
 #include "f2fs.h"
 #include "segment.h"