No need of using `M2_HOME` environment variable as fallback to find path to maven executable in Windows

We no longer need to get the path to maven executable [via `M2_HOME`](https://github.com/INRIA/spoon/blob/master/src/main/java/spoon/MavenLauncher.java#L93) because its usage has been deprecated with [newer Windows releases](https://github.com/actions/virtual-environments/issues/4485#issuecomment-967145878).

EDIT:
Support for `M2_HOME` was deprecated in Maven 3.5.0. See [5th point in the overview about the changes](https://maven.apache.org/docs/3.5.0/release-notes.html#overview-about-the-changes).