[202405] Remove onboarding PR test job (#19856)

Currently we are adding more control plane and data plane test to PR test in master branch, and we added onboarding test job for piloting. When we are working on this goal, 202405 branch was created with onboarding test job, but it's not needed.

azure-pipelines.yml

@@ -261,22 +261,6 @@ stages:
           MAX_WORKER: $(T0_SONIC_INSTANCE_NUM)
           MGMT_BRANCH: $(BUILD_BRANCH)
 
-  - job: onboarding_elastictest_t0
-    displayName: "onboarding testcases by Elastictest"
-    timeoutInMinutes: 240
-    continueOnError: true
-    pool: sonic-ubuntu-1c
-    steps:
-      - template: .azure-pipelines/run-test-elastictest-template.yml@sonic-mgmt
-        parameters:
-          TOPOLOGY: t0
-          MIN_WORKER: $(T0_ONBOARDING_SONIC_INSTANCE_NUM)
-          MAX_WORKER: $(T0_ONBOARDING_SONIC_INSTANCE_NUM)
-          KVM_IMAGE_BRANCH: $(BUILD_BRANCH)
-          MGMT_BRANCH: $(BUILD_BRANCH)
-          TEST_SET: onboarding_t0
-
-
 #  - job: wan_elastictest
 #    displayName: "kvmtest-wan by Elastictest"
 #    pool: sonic-ubuntu-1c