systemd: simplify unit file

Avoid setting up the Alias rule, which was a helper to let us 'disable' the
systemd service including dbus-activations. Without the Alias, 'disable' will
still let starting ModemManager through dbus-activation. If you really want to
fully disallow starting MM also through dbus-activation, you should 'mask' and
'unmask' the service.

E.g.:
 $ sudo systemctl mask ModemManager
ln -s '/dev/null' '/etc/systemd/system/ModemManager.service'

 $ sudo mmcli -L
error: couldn't find the ModemManager process in the bus

 $ sudo systemctl unmask ModemManager
rm '/etc/systemd/system/ModemManager.service'

 $ sudo mmcli -L
No modems were found

https://bugzilla.gnome.org/show_bug.cgi?id=701229

data/ModemManager.service.in

@@ -10,4 +10,3 @@ StandardError=null
 
 [Install]
 WantedBy=multi-user.target
-Alias=dbus-org.freedesktop.ModemManager1.service