mm/writeback: remove wb_reason_name

wb_reason_name is not used any more - remove it.
Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Reviewed-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/writeback.h

@@ -51,7 +51,6 @@ enum wb_reason {
 
 	WB_REASON_MAX,
 };
-extern const char *wb_reason_name[];
 
 /*
  * A control structure which tells the writeback code what to do.  These are