Drop the SystemdService line in dbus service files

I don't think we can dbus activate systemd services on
the session bus (we don't have a user bus, and I'm
pretty sure we don't have systemd --user)

This commit just drops the lines, so dbus can activate on
its own.
diff -up flatpak-0.8.8/session-helper/org.freedesktop.Flatpak.service.in.no-user-systemd flatpak-0.8.8/session-helper/org.freedesktop.Flatpak.service.in
--- flatpak-0.8.8/session-helper/org.freedesktop.Flatpak.service.in.no-user-systemd	2017-11-08 16:33:21.045226761 -0500
+++ flatpak-0.8.8/session-helper/org.freedesktop.Flatpak.service.in	2017-11-08 16:33:38.361037649 -0500
@@ -1,4 +1,3 @@
 [D-BUS Service]
 Name=org.freedesktop.Flatpak
 Exec=@libexecdir@/flatpak-session-helper
-SystemdService=flatpak-session-helper.service
--- flatpak-1.0.1/portal/org.freedesktop.portal.Flatpak.service.in.no-user-systemd	2018-05-30 14:58:57.000000000 +0200
+++ flatpak-1.0.1/portal/org.freedesktop.portal.Flatpak.service.in	2018-09-12 12:11:27.428215430 +0200
@@ -1,4 +1,3 @@
 [D-BUS Service]
 Name=org.freedesktop.portal.Flatpak
 Exec=@libexecdir@/flatpak-portal
-SystemdService=flatpak-portal.service