x86: simplify (and fix) clear_IO_APIC{,_pin}()

x86: simplify (and fix) clear_IO_APIC{,_pin}()

These are used during bootup and (emergency) shutdown only, and their
only purpose is to get the actual IO-APIC's RTE(s) cleared.
Consequently, only the "raw" accessors should be used (and the ones
going through interrupt remapping code can be skipped), with the
exception of determining the delivery mode: This one must always go
through the interrupt remapping path, as in the VT-d case the actual
IO-APIC's RTE will have the delivery mode always set to zero (which
before possibly could have resulted in such an entry getting cleared
in the "raw" pass, though I haven't observed this case in practice).

Signed-off-by: Jan Beulich <jbeulich@novell.com>