needs-restarting: Revert using systemd start time

Since commit a0ac871 "dnf
needs-restarting -s" failed to spot services started withing few
seconds before updating their RPM package.

The cause was using systemd start time instead of a kernel boot time
when computing an absolute start time of processes.

This patch removes using the systemd start time until a proper fix is
found.

https://issues.redhat.com/browse/RHEL-39775

doc/needs_restarting.rst

@@ -36,12 +36,8 @@ Description
 Note that in most cases a process should survive update of its binary and libraries it is using without requiring to be restarted for proper operation. There are however specific cases when this does not apply. Separately, processes often need to be restarted to reflect security updates.
 
 .. note::
-   Needs-restarting will try to guess the boot time using three different methods:
+   Needs-restarting will try to guess the boot time using two different methods:
 
-   ``UnitsLoadStartTimestamp``
-        D-Bus property on ``/org/freedesktop/systemd1``.
-        Works unless the system was not booted with systemd,
-        such as in (most) containers.
    ``st_mtime of /proc/1``
         Reflects the time the first process was run after booting.
         This works for all known cases except machines without

plugins/needs_restarting.py

@@ -218,11 +218,6 @@ def get_boot_time():
         We have two sources from which to derive the boot time. These values vary
         depending on containerization, existence of a Real Time Clock, etc.
         For our purposes we want the latest derived value.
-        - UnitsLoadStartTimestamp property on /org/freedesktop/systemd1
-             The start time of the service manager, according to systemd itself.
-             Seems to be more reliable than UserspaceTimestamp when the RTC is
-             in local time. Works unless the system was not booted with systemd,
-             such as in (most) containers.
         - st_mtime of /proc/1
              Reflects the time the first process was run after booting
              This works for all known cases except machines without
@@ -233,25 +228,6 @@ def get_boot_time():
              Does not work on containers which share their kernel with the
              host - there the host kernel uptime is returned
         """
-        units_load_start_timestamp = None
-        try:
-            bus = dbus.SystemBus()
-            systemd1 = bus.get_object(
-                'org.freedesktop.systemd1',
-                '/org/freedesktop/systemd1'
-            )
-            props = dbus.Interface(
-                systemd1,
-                dbus.PROPERTIES_IFACE
-            )
-            units_load_start_timestamp = props.Get(
-                'org.freedesktop.systemd1.Manager',
-                'UnitsLoadStartTimestamp'
-            )
-            if units_load_start_timestamp != 0:
-                return int(units_load_start_timestamp / (1000 * 1000))
-        except dbus.exceptions.DBusException as e:
-            logger.debug("D-Bus error getting boot time from systemd: {}".format(e))
 
         proc_1_boot_time = int(os.stat('/proc/1').st_mtime)
         if os.path.isfile('/proc/uptime'):