Remove `cta-versionlock` script and extract `cta-checkpkg` script

The cta-versionlock script still uses Python2 and is no longer necessary. It was primarily used to updated the existing versionlock when switching between eos 4/5 and xroot 4/5. As we are now on v5 of both of these there is no reason to keep it. The only thing that is still useful here is the checkpkg command. This checks whether the packages are correctly installed according to the versionlock. This functionality should be extracted into a separate Python3 script.

This script is not necessary for end-user, it is mainly for the ci (or potentially extra peace of mind)