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 (closed)

[1] https://github.com/systemd/systemd/pull/5600

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