Revert "systemd-logind: Monitor systemd-logind going away"

systemd-logind since version 234 (released 2017-07-12) supports being
restarted without losing state [1]. From the systemd NEWS file [2]:

 * systemd-logind may now be restarted without losing state. It stores
   the file descriptors for devices it manages in the system manager
   using the FDSTORE= mechanism. Please note that further changes in
   other components may be required to make use of this (for example
   Xorg has code to listen for stops of systemd-logind and terminate
   itself when logind is stopped or restarted, in order to avoid using
   stale file descriptors for graphical devices, which is now
   counterproductive and must be reverted in order for restarts of
   systemd-logind to be safe. See
   https://cgit.freedesktop.org/xorg/xserver/commit/?id=dc48bd653c7e101.)

This reverts commit dc48bd65.

Closes: #531

[1] https://github.com/systemd/systemd/pull/5600
[2] https://github.com/systemd/systemd/commit/9f09a95a7e6127f526c0ef7b6fd060cc0b01cdcc
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