Remove outdated puppet_agent pin

Author: LukasAud

.fixtures.yml

@@ -4,7 +4,6 @@ fixtures:
     provision: 'https://github.com/puppetlabs/provision.git'
     puppet_agent:
       repo: 'https://github.com/puppetlabs/puppetlabs-puppet_agent.git'
-      ref: v4.21.0
     stdlib: 'https://github.com/puppetlabs/puppetlabs-stdlib.git'
   symlinks:
     concat: '#{source_dir}'