KVM: Drop sanity check that per-VM list of irqfds is unique

KVM: Drop sanity check that per-VM list of irqfds is unique

Now that the eventfd's waitqueue ensures it has at most one priority
waiter, i.e. prevents KVM from binding multiple irqfds to one eventfd,
drop KVM's sanity check that eventfds are unique for a single VM.

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250522235223.3178519-11-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>