Remove hack which builds the IO package

[fingolfin]

This was/is a workaround for issues in RepnDecomp and HeLP and maybe other packages that went undetected so far.

Should not be merged until fixes for those issues are in releases of those packages:

• RepnDecomp
  • issue: Syntax warnings when RepnDecomp is loaded without IO gap-packages/RepnDecomp#23
  • PR: Avoid syntax warnings when IO is not loaded gap-packages/RepnDecomp#24
  • not yet fixed in a release
• HeLP
  • issue: Loading HeLP without NormalizInterface or 4ti2Interface results in syntax warnings gap-packages/HeLP#22
  • no PR fixing this available yet
  • not yet fixed in a release

[wilfwilson]

I've made an issue asking the developers/maintainers of RepnDecomp to kindly make a release 🙂

[wilfwilson]

I don't think we actually have to wait for HeLP to get fixed; the issue with HeLP only appeared when we introduced this hack, I believe.

Of course, the issue with HeLP still exists, but I don't think it will affect our CI in gap-actions/run-pkg-tests#45 if this PR is merged.