CLEANUP: fd: finally get rid of fd_done_recv()

CLEANUP: fd: finally get rid of fd_done_recv()

fd_done_recv() used to be useful with the FD cache because it used to
allow to keep a file descriptor active in the poller without being
marked as ready in the cache, saving it from ringing immediately,
without incurring any system call. It was a way to make it yield
to wait for new events leaving a bit of time for others. The only
user left was the connection accepter (listen_accept()). We used
to suspect that with the FD cache removal it had become totally
useless since changing its readiness or not wouldn't change its
status regarding the poller itself, which would be the only one
deciding to report it again.

Careful tests showed that it indeed has exactly zero effect nowadays,
the syscall numbers are exactly the same with and without, including
when enabling edge-triggered polling.

Given that there's no more API available to manipulate it and that it
was directly called as an optimization from listener_accept(), it's
about time to remove it.