Changes

{contrib,debian}/osmo-ggsn.init: remove

Remove SysV init scripts. These are not really maintained anymore and
this makes it consistent with other Osmocom projects.

Avoids synchronizing with SysV scripts on debian:
  # systemctl enable osmo-ggsn
  Synchronizing state of osmo-ggsn.service with SysV service script with /lib/systemd/systemd-sysv-install.
  Executing: /lib/systemd/systemd-sysv-install enable osmo-ggsn

Change-Id: I11bfb5122344b54970b7f742470cb74b95fa37e0