irqchip/gic: Rely on MSI default .irq_eoi()

Previously, only platform-MSI irqchips would get a default .irq_eoi().
GIC-based platform-MSI irqchip's rely on this default callback, while
PCI-MSI ones are initialized explicitely.

As all MSI domains now get a default .irq_eoi(), drop the explicit
.irq_eoi() initialization for PCI-MSI chips.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>