Drop support for CI builds using Azure Pipelines (git-for-windows#5389)

A long time ago, we used Azure Pipelines for CI builds. Eventually, when
GitHub Actions was spawned from Azure Pipelines, we moved over to that
system for CI/PR builds.

However, I've tried to keep a working Azure Pipelines definition around
for the embargoed releases, to be able to increase confidence in the
patches by running the full CI build in a private Azure DevOps project
(we cannot do the same on GitHub because even the very generous offer of
50,000 build minutes per month is no match for Git's test suite).

It took a lot of work, and it would take even more work what with
upstream's "remove stale code for Azure Pipelines" part of
`ps/ci-misc-updates`.

Time to let it go.

Author: dscho