Remove CentOS CI pipeline (#6997)

tools/ci_build/github/azure-pipelines/centos-ci-pipeline.yml

@@ -1,26 +0,0 @@
-jobs:
-- job: linux_centos_ci
-  workspace:
-    clean: all
-  timeoutInMinutes:  60
-  pool: 'Linux-CPU-2019'
-  strategy:
-    maxParallel: 2
-    matrix: 
-      Debug:
-        BuildType: Debug
-      Release:
-        BuildType: Release
-  steps:
-    - task: CmdLine@2
-      inputs:
-        script: |
-          docker build --pull -t onnxruntime-centos7 --build-arg BUILD_USER=onnxruntimedev --build-arg BUILD_UID=$(id -u) --build-arg PYTHON_VERSION=3.6 -f Dockerfile.centos .
-        workingDirectory: $(Build.SourcesDirectory)/tools/ci_build/github/linux/docker
-    - task: CmdLine@2
-      inputs:
-        script: |
-          docker run --rm --volume /data/onnx:/data/onnx:ro --volume $(Build.SourcesDirectory):/onnxruntime_src --volume $(Build.BinariesDirectory):/build -e NIGHTLY_BUILD onnxruntime-centos7 /usr/bin/python3 /onnxruntime_src/tools/ci_build/build.py --build_dir /build --config $(BuildType) --skip_submodule_sync  --parallel --build_shared_lib --use_openmp --cmake_path /usr/bin/cmake --ctest_path /usr/bin/ctest --build_wheel
-        workingDirectory: $(Build.SourcesDirectory)
-
-    - template: templates/clean-agent-build-directory-step.yml