testing/xonotic: remove unused deps

- modplug has not been used since [0] over 8 years ago
- libvorbis has not been used since at least git conversion in 2010.
  vorbis-tools is used for some auxiliary scripts
- xf86dga has not been used since at least git conversion in 2010

[0] https://gitlab.com/xonotic/xonotic/-/commit/3b7a0fb43d8ae37edcba84e6cbe44ca584cdc940