irqchip/irq-mvebu-icu: Remove platform MSI leftovers

All related domains provide MSI parent functionality, so the fallback code
to the original platform MSI implementation is not longer required.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Shivamurthy Shastri <shivamurthy.shastri@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240623142235.881677325@linutronix.de