Revert "Extend CI intake pipeline to run BC upgrade tests (elastic#128614)" (elastic#129525)

This reverts commit e879299.

Author: ldematte

.buildkite/pipelines/intake.template.yml

@@ -72,8 +72,6 @@ steps:
           buildDirectory: /dev/shm/bk
         env:
           BWC_VERSION: "{{matrix.BWC_VERSION}}"
-  - label: bc-upgrade
-    command: ".buildkite/scripts/run-bc-upgrade-tests.sh"
   - group: lucene-compat
     steps:
       - label: "{{matrix.LUCENE_VERSION}} / lucene-compat"

.buildkite/pipelines/intake.yml

@@ -72,11 +72,6 @@ steps:
           buildDirectory: /dev/shm/bk
         env:
           BWC_VERSION: "{{matrix.BWC_VERSION}}"
-  - label: bc-upgrade
-    command: ".buildkite/scripts/run-bc-upgrade-tests.sh"
-    agents:
-      image: "docker.elastic.co/ci-agent-images/eck-region/buildkite-agent:1.5"
-      memory: "4G"
   - group: lucene-compat
     steps:
       - label: "{{matrix.LUCENE_VERSION}} / lucene-compat"