net: caif: 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 caif at all.

So, remove the unused hardirq.h.

Signed-off-by: Yang Shi <yang.s@alibaba-inc.com>
Cc: Dmitry Tarnyagin <dmitry.tarnyagin@lockless.no>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>