MINOR: activity/fd: remove the dead_fd counter

MINOR: activity/fd: remove the dead_fd counter

This one is set whenever an FD is reported by a poller with a null owner,
regardless of the thread_mask. It has become totally meaningless because
it only indicates a migrated FD that was not yet reassigned to a thread,
but as soon as a thread uses it, the status will change to skip_fd. Thus
there is no reason to distinguish between the two, it adds more confusion
than it helps. Let's simply drop it.