service: sysv - remove distribution specific targets

Systemd should not introduce any new facilities. Distributions which still
need to support their non-standard/legacy facilities should add them as
patches to their packaging.

The following facilities are no longer recognized:
  $x-display-manager
  $mail-transfer-agent
  $mail-transport-agent
  $mail-transfer-agent
  $smtp
  $null

This target is no longer available:
  mail-transfer-agent.target