generator: Drop fallback mechanism (OnFailure=nonisuNAME.service)

generator: Drop fallback mechanism (OnFailure=nonisuNAME.service)

Unfortunately, the fallback mechanism, used when ISU service fails
to start can not be used in practice because:

 - it breaks socket activated services, as socket passing is defined
   in .socket units explicitly (ie. .socket specifies eg. Service=a.service,
   and if fallaback uinit would be started then sockets would not be
   passed correctly),

 - it breaks dbus-activated services, as copying original service file
   to nonisuNAME.service causes two services to specify same BusName= name

Change-Id: I7dc187ff367920bc2bfcf3988506f513278a932a