ci: retire the Azure Pipelines definition

We have GitHub Actions now. Running the same builds and tests in Azure
Pipelines would be redundant, and a waste of energy.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

README.md

@@ -1,5 +1,4 @@
 [![Build status](https://github.com/git/git/workflows/CI/PR/badge.svg)](https://github.com/git/git/actions?query=branch%3Amaster+event%3Apush)
-[![Build Status](https://dev.azure.com/git/git/_apis/build/status/git.git)](https://dev.azure.com/git/git/_build/latest?definitionId=11)
 
 Git - fast, scalable, distributed revision control system
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