Cron Job of auto-prune not getting deleted when user reverts pruner config  to default in tektonconfig

Steps to Reproduce:

1. Install Openshift Pipelines 1.5
2. Update config CR with the below values

pruner:
    keep: 2
    resources:
      - pipelinerun
    schedule: '*/5 * * * *'

1. Check if the cronjob got created in namepsace `openshift-pipelines`
2. After sometime try to revert change to default in tektonconfig

   pruner: 
      keep: 1
      schedule: ''
   

Expected Behaviour:

• It should remove cronjob on openshift-pipelines namespace

Actual Behaviour:

• But cronjob still continues with older configuration.