drbd: remove duplicated #include

Remove duplicated #include('s) in
  drivers/block/drbd/drbd_worker.c
Signed-off-by: Huang Weiyi <weiyi.huang@gmail.com>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>

drivers/block/drbd/drbd_worker.c

@@ -34,7 +34,6 @@
 #include <linux/mm_inline.h>
 #include <linux/slab.h>
 #include <linux/random.h>
-#include <linux/mm.h>
 #include <linux/string.h>
 #include <linux/scatterlist.h>