fix duplication route prefix in RestCollectionLoader

The prefix is included in the import resource:
`$imported = $this->processor->importResource($this, $resource, $parents, $prefix, $namePrefix, $type, $currentDir);`

In the line
`$imported->addPrefix($prefix);`
prefix again added

Generated route:
`/api/foo/foo/bar`
instead:
`/api/foo/bar`