um/irqstat: Get rid of the duplicated declarations

irq_cpustat_t and ack_bad_irq() are exactly the same as the asm-generic
ones.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20201113141733.156361337@linutronix.de