irqdomain: Remove __irq_domain_add()

__irq_domain_add() has been replaced by irq_domain_instanciate() and so,
it is no more used.

Simply remove it.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240614173232.1184015-21-herve.codina@bootlin.com