arch/x86: eliminate include/arch/x86/irq_controller.h

The MVIC is no longer supported, and only the APIC-based interrupt
subsystem remains. Thus this layer of indirection is unnecessary.

This also corrects an oversight left over from the Jailhouse x2APIC
implementation affecting EOI delivery for direct ISRs only.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>