x86/IRQ: simplify pending EOI stack logic for internally used IRQs

x86/IRQ: simplify pending EOI stack logic for internally used IRQs

In 5655ce8b1ec2 ("x86/IRQ: make internally used IRQs also honor the
pending EOI stack") it was mentioned that both the check_eoi_deferral
per-CPU variable and the cpu_has_pending_apic_eoi() were added just to
have as little impact on existing behavior as possible, to reduce the
risk of a last minute regression in 4.13.

Upon closer inspection, dropping the variable is an option only if all
callers of ->end() would assume the responsibility of also calling
flush_ready_eoi(). Therefore only drop the cpu_has_pending_apic_eoi()
guard now.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>