[libs.segmentation.scripts.evaluate] Fix unreachable code

src/mednet/libs/segmentation/scripts/evaluate.py

@@ -32,9 +32,6 @@ def validate_threshold(threshold: float | str, splits: list[str]):
     -------
         The validated threshold.
     """
-    if threshold is None:
-        return 0.5
-
     try:
         # we try to convert it to float first
         threshold = float(threshold)