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>
Link: https://lore.kernel.org/r/YidLo57Kw/u/cpA5@linutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>