remove outdated pin

Author: LukasAud

.fixtures.yml

@@ -10,5 +10,4 @@ fixtures:
     facts: 'https://github.com/puppetlabs/puppetlabs-facts.git'
     puppet_agent:
       repo: 'https://github.com/puppetlabs/puppetlabs-puppet_agent.git'
-      ref: v4.21.0
     provision: 'https://github.com/puppetlabs/provision.git'