[MNG-7260] Remove old deprecated code

**[Guillaume Nodet](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=gnt)** opened **[MNG-7260](https://issues.apache.org/jira/browse/MNG-7260?redirect=false)** and commented

Things like `maven-compat` for example, but we should review everything that has been deprecated since quite some time and possibly remove those.

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**Sub-tasks:**
- [MNG-7266](https://issues.apache.org/jira/browse/MNG-7266) Remove maven-compat module

**Issue Links:**
- [MNG-7266](https://issues.apache.org/jira/browse/MNG-7266) Remove maven-compat module
(_**"incorporates"**_)
- [MNG-7007](https://issues.apache.org/jira/browse/MNG-7007) Remove documented backward compatibility

**Remote Links:**
- [GitHub Pull Request #552
](https://github.com/apache/maven/pull/552)