This reverts commit 91898aa8b0.
See additional comments in the following bug:
https://bugzilla.gnome.org/show_bug.cgi?id=701229
Basically, 'mask' and 'unmask' operations are not the ones we should be using
or suggesting; and the Alias= for the DBus file is the correct way to go.
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
