x86/irq: Move apic_post_init() invocation to one place

No point to call it from both 32bit and 64bit implementations of
default_setup_apic_routing(). Move it to the caller.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200826112332.658496557@linutronix.de