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

@@ -38,10 +38,6 @@ Note that in most cases a process should survive update of its binary and librar
 .. note::
    Needs-restarting will try to guess the boot time using three 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'):