Remove PostBuildCleanup (#23233)
Remove PostBuildCleanup tasks since it is deprecated. It is to address a
warning in our pipelines:
"Task 'Post Build Cleanup' version 3 (PostBuildCleanup@3) is dependent
on a Node version (6) that is end-of-life. Contact the extension owner
for an updated version of the task. Task maintainers should review Node
upgrade guidance: https://aka.ms/node-runner-guidance"
Now the cleanup is controlled in another place:
https://learn.microsoft.com/en-us/azure/devops/pipelines/yaml-schema/workspace?view=azure-pipelines
The code change was generated by the following Linux command:
```bash
find . -name \*.yml -exec sed -i '/PostBuildCleanup/,+2d' {} \;
```
