Graceful fallback in Filesystem::removeDirectory() to php

On a shared system, we have some "Could not completely delete" problems, like this:

```
Loading composer repositories with package information
Updating dependencies (including require-dev)
 
- Removing menatwork/contao-multicolumnwizard (3.1.0)
 
  *Internals*
    command: string(80) "rm -rf '/.../vendor/menatwork/contao-multicolumnwizard'"
    return status: bool(false)
    directory still exists: bool(true)
    error output: string(31) "sh: /bin/rm: Permission denied "
 
[RuntimeException]
Could not completely delete /.../vendor/menatwork/contao-multicolumnwizard, aborting.
```

`Filesystem::removeDirectory()` is the only function in `Filesystem`, that does not do a graceful fallback to php, if the process fails. Now it does :-)