Makefile: Remove cleanup of ruby/

We deleted a bunch of files and directories related to the Ruby sidecar, but some of them were mentioned in .gitignore. This caused git to never remove them. For that reason we introduced a target clean-ruby which was called by build and clean to make sure these files get deleted some way or the other.

Because that target has been around for a couple of releases now, it should have done it's duty, and it's safe to retire it.

Milestone: 16.3 Label: maintenance::removal