Remove unused Ruby versions
Remove unused Ruby versions

These were found by running `fab production puppet_class:govuk_rbenv::all rbenv.version_in_use`
for each Ruby version and checking the output.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet