KVM: x86: remove the unused assigned_dev_head from kvm_arch

Legacy device assignment was dropped years ago. This field is not used
anymore.

Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
Link: https://lore.kernel.org/r/20231019043336.8998-1-liangchen.linux@gmail.com


Signed-off-by: Sean Christopherson <seanjc@google.com>