Remove jenkins.build.ref.guide.sh

After the move to Cloudbees (ci-builds.apache.org), the Ref Guide Jenkins jobs stopped working. The dev-tools/scripts/jenkins.build.ref.guide.sh script we used to build the Guide installed its own RVM and gemset for the required gems to run with the Ant build and it was difficult to get the paths right. Infra added the dependencies that we need to their Puppet-managed node deploy process (see INFRA-20656) and now we don't need a script to do any of that for us.

This issue is to track removing the script since it's no longer required. The Ref Guide build jobs will just invoke Ant directly instead.

IIUC from SOLR-10568 when the script was added, there might still come a day when there is a version mismatch between what was installed by default and what our build needs, but I think it's fair to try to work with Infra to get our needs met on the nodes instead of adding them to a script which makes migration like this more complex.

All of these pre-build dependencies go away, however, when we move to Gradle, so even if we have a version mismatch one time it won't be a persistent issue.