logind: drop session fifo logic, rely solely on pidfd for exit notification

logind: drop session fifo logic, rely solely on pidfd for exit notification

Traditionally, logind installed a fifo in the PAM session and
used EOF on the fd as signal for session close. With the addition of
pidfd (76f2191d8eb54d7b9e39ab230c9c62b8a8c42265) however,
logind tracks the leader process and the session is terminated
as soon as that exits. I think the new behavior generally makes
more sense, and the behavior got changed *in the mentioned commit
already* without anyone ever showing up to complain. It hence
feels safe to kill the concept now (also before the varlink interface
gets rolled out).

Note that the 'PID' field in CreateSession() Varlink method
is now marked as strict, i.e. failure to acquire pidfd
is immediately treated as fatal.