Fix parsing of labels.json when generating stats

The structure of the labels.json used in unit tests is not up to date. There is no "text" key anymore.

{
    "split_name": {
        "path/to/image1": "label",
        ...
    }
}

dan/datasets/analyze/statistics.py

 # -*- coding: utf-8 -*-
 from collections import Counter, defaultdict
-from operator import itemgetter
 from pathlib import Path
 from typing import Dict, List, Optional
 
@@ -156,8 +155,7 @@ class Statistics:
             # Path to the images are the key of the dict
             self.create_image_statistics(images=split_data.keys())
 
-            # The text is actually under the "text" key of the values
-            labels = list(map(itemgetter("text"), split_data.values()))
+            labels = list(split_data.values())
             # Text statistics
             self.create_label_statistics(labels=labels)