Remove onboarding PR test job (#22196)

Why I did it
Remove onboarding PR test job was using for adding more PR test scripts to PR test set, but now mgmt branch already supported all PR test scripts with pytest mark and conditional mark, onboarding PR test jobs are no longer needed.

Work item tracking
Microsoft ADO (number only):
How I did it
Remove onboarding PR test jobs to save more instances.

Author: xwjiang-ms

azure-pipelines.yml

@@ -264,57 +264,6 @@ stages:
           MAX_WORKER: $(T0_SONIC_INSTANCE_NUM)
           MGMT_BRANCH: $(BUILD_BRANCH)
 
-  - job: onboarding_elastictest_t0
-    displayName: "onboarding t0 testcases by Elastictest - optional"
-    timeoutInMinutes: 240
-    continueOnError: true
-    pool: sonic-ubuntu-1c
-    steps:
-      - template: .azure-pipelines/run-test-elastictest-template.yml@sonic-mgmt
-        parameters:
-          TOPOLOGY: t0
-          STOP_ON_FAILURE: "False"
-          RETRY_TIMES: 0
-          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: onboarding_elastictest_t1
-    displayName: "onboarding t1 testcases by Elastictest - optional"
-    timeoutInMinutes: 240
-    continueOnError: true
-    pool: sonic-ubuntu-1c
-    steps:
-      - template: .azure-pipelines/run-test-elastictest-template.yml@sonic-mgmt
-        parameters:
-          TOPOLOGY: t1-lag
-          STOP_ON_FAILURE: "False"
-          RETRY_TIMES: 0
-          MIN_WORKER: $(T1_LAG_ONBOARDING_INSTANCE_NUM)
-          MAX_WORKER: $(T1_LAG_ONBOARDING_INSTANCE_NUM)
-          KVM_IMAGE_BRANCH: $(BUILD_BRANCH)
-          MGMT_BRANCH: $(BUILD_BRANCH)
-          TEST_SET: onboarding_t1
-
-#  - job: onboarding_elastictest_dualtor
-#    displayName: "onboarding dualtor testcases by Elastictest - optional"
-#    timeoutInMinutes: 240
-#    continueOnError: true
-#    pool: sonic-ubuntu-1c
-#    steps:
-#      - template: .azure-pipelines/run-test-elastictest-template.yml@sonic-mgmt
-#        parameters:
-#          TOPOLOGY: dualtor
-#          STOP_ON_FAILURE: "False"
-#          RETRY_TIMES: 0
-#          MIN_WORKER: $(T0_DUALTOR_INSTANCE_NUM)
-#          MAX_WORKER: $(T0_DUALTOR_INSTANCE_NUM)
-#          KVM_IMAGE_BRANCH: $(BUILD_BRANCH)
-#          MGMT_BRANCH: $(BUILD_BRANCH)
-#          TEST_SET: onboarding_dualtor
-
 #  - job: wan_elastictest
 #    displayName: "kvmtest-wan by Elastictest"
 #    pool: sonic-ubuntu-1c