ci!: remove clas12-validation trigger, in favor of orca (#1111)

[`orca`](https://code.jlab.org/hallb/clas12/orca) is now running more
smoothly, and is meant to replace [`clas12-validation`](https://github.com/JeffersonLab/clas12-validation).
We've been running both for some time, now it's time to disable our
`clas12-validation` trigger.

Author: c-dilks