Revert "BUG/MEDIUM: listener: do not accept connections faster than we can process...

Revert "BUG/MEDIUM: listener: do not accept connections faster than we can process them"

This reverts commit 62e8aaa1bd5ca96089eaa88487c700c4af4617f4.

While is works extremely well to address SSL handshake floods, it prevents
establishment of new connections during regular traffic above 50-60 Gbps,
because for an unknown reason the queue seems to have ~1.7 active tasks
per connection all the time, which makes no sense as these ought to be
waiting on subscribed events. It might uncover a deeper issue but at least
for now a different solution is needed. cf issue #822.

The test is trivial to run, just start a config with tune.runqueue-depth 10
and inject on 1GB objects with more than 10 connections. Try to connect to
the stats socket, it only works once, then the listeners are not dequeued.