x86/APIC: drop probe_default()

x86/APIC: drop probe_default()

The function does nothing but return success. Simply treat absence of a
probe hook to mean just this. This then eliminates the (purely
theoretical at this point) risk of trying to call through
apic_x2apic_{cluster,phys}'s respective NULL pointers.

While doing this also eliminate generic_apic_probe()'s "changed"
variable: apic_probe[]'s default entry will now be used unconditionally
in yet more obvious a way, such that separately setting genapic from
apic_default is (hopefully) no longer justified. Yet that was the main
purpose of the variable.

To help prove that apic_default's probe() hook doesn't get used
elsewhere, further make apic_probe[] static at this occasion.

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