genirq: Get rid of GENERIC_MSI_IRQ_DOMAIN

Adjust to reality and remove another layer of pointless Kconfig
indirection. CONFIG_GENERIC_MSI_IRQ is good enough to serve
all purposes.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20221111122014.524842979@linutronix.de