btrfs: remove unused hardirq.h

Preempt counter APIs have been split out, currently, hardirq.h just
includes irq_enter/exit APIs which are not used by btrfs at all.

So, remove the unused hardirq.h.
Signed-off-by: Yang Shi <yang.s@alibaba-inc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_map.c

@@ -2,7 +2,6 @@
 #include <linux/err.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
-#include <linux/hardirq.h>
 #include "ctree.h"
 #include "extent_map.h"
 #include "compression.h"