Remove the last call to Gecode::Gist

[pierrewillenbrockdfki]

Gist is the Gecode Interactive Search Tool, using Qt to display a GUI.

This means the templ library depends on the qt version gecode is built with if Gecode::Gist is referenced anywhere, which poses a problem when it is to be used with a different qt version.

[pierrewillenbrockdfki]

@2maz judging by the commented calls to Gecode::Gist, there seems to have been some effort to remove it already. Also, i cannot see how an interactive search tool would have a place in the non-gui library bits.

I am planning to remove Gecode::Gist and Gecode::Flatzinc from knowledge_reasoning/moreorg(where none of them is used either), so the reference here has to be removed first.

I'll be on vacation for a week and merge this afterwards if there are no problems/comments/it is not already merged.

[2maz]

Fine with me. Although I suggest to remove the reference then entirely. Not just commenting.

[pierrewillenbrockdfki]

Looks like the runs-on: ubuntu-18.04 has finally been removed. I'll do another PR that fixes that, merge that here and then things should be back to working.