Remove the `autogen` command from `dunecontrol`.  After the removal of
autotools support, running `dunecontrol autogen` would just return an
error anyway:

```
    $ ./dunecontrol --only=dune-common autogen
    [...]
    ./dune-common/bin/dunecontrol: line 341: run_default_autogen: command not found
    [...]
```

In addition the `dune-autogen` script was already removed in
4935e077.