Forced commit to note that the hack removed in the previous commit

really used to be necessary.  bus_teardown_interrupt() was completely
broken for fast interrupts in -current from approx. 2001/02/09 to
2003/11/03.  It not only didn't shut down the hardware interrupt; it
also left the fast interrupt handler wired into the IDT.  The hack was
needed to shut down the hardware interrupt.  Without it, for npx,
unmasked exceptions were delivered via both IRQ13 and Exception16, and
spurious IRQ13 broke exception handling in much the same way that it
is broken on old systems that don't support Exception16.

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