debian/osmo-gtphub.init: delete

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-gtphub
  Synchronizing state of osmo-gtphub.service with SysV service script with /usr/lib/systemd/systemd-sysv-install.
  Executing: /usr/lib/systemd/systemd-sysv-install enable osmo-gtphub

Change-Id: I9008944369314a4cbb345bfbf01bdb57aa7590fb