ci: let `clas12-validation` set the GEMC `ref`
Created by: c-dilks
https://github.com/JeffersonLab/clas12-validation/pull/68 will be setting the default ref
for each upstream repo, so let's remove the override here.
This PR won't work until https://github.com/JeffersonLab/clas12-validation/pull/68 is merged.