virt: acrn: Remove unsued acrn_irqfds_mutex.

acrn_irqfds_mutex is not used, never was.

Remove acrn_irqfds_mutex.

Fixes: aa3b483f ("virt: acrn: Introduce irqfd")
Cc: Fei Li <fei1.li@intel.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>