remove tf plan workflow from running on pr

.github/workflows/deployment_plan.yaml

@@ -2,13 +2,13 @@ name: Terraform Plan
 run-name: Terraform plan ${{ inputs.workspace }} by @${{ github.actor }}
 
 on:
-  pull_request:
   workflow_dispatch:
     inputs:
       workspace:
         description: 'The workspace to terraform against'
         required: true
         type: choice
+        default: prod
         options:
           - ""
           - prod