KVM: arm/arm64: Delete outdated forwarded irq documentation

The reason I added this documentation originally was that the concept of
"never taking the interrupt", but just use the timer to generate an exit
from the guest, was confusing to most, and we had to explain it several
times over.  But as we can clearly see, we've failed to update the
documentation as the code has evolved, and people who need to understand
these details are probably better off reading the code.

Let's lighten our maintenance burden slightly and just get rid of this.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>