[v1.10.x-aws] .ci/aws: Remove unnecessary stages

deleteDir() recursively removes the Jenkins provided workspace on the
EFS. We run PortaFiducia out of local controller ephemeral storage,
which gets deleted with each job. Since we are not using the workspace,
there is no need to clean it up before each job.

checkoutSCMRepo pulls down aws-ofi-nccl to the fargate agent, which is a
waste of time because we only need it on the cluster.

Signed-off-by: Seth Zegelstein <szegel@amazon.com>
(cherry picked from commit a8009b8)

.ci/aws/Jenkinsfile

@@ -140,17 +140,6 @@ pipeline {
         REGION="us-west-2"
     }
     stages {
-        // Cleanup workspace before job start.
-        stage("Clean up workspace") {
-            steps{
-                deleteDir()
-            }
-        }
-        stage("Checkout SCM repo") {
-            steps {
-                checkout scm
-            }
-        }
         stage("Download and extract PortaFiducia") {
             steps {
                 script {