Remove non-used z-score function

Author: jaime-cespedes-sisniega

frouros/utils/stats.py

@@ -263,22 +263,3 @@ def permutation(  # pylint: disable=too-many-arguments,too-many-locals
         ).get()
 
     return permuted_statistics
-
-
-def z_score(
-    value: np.ndarray,
-    mean: float,
-    std: float,
-) -> np.ndarray:
-    """Z-score method.
-
-    :param value: value to use to compute the z-score
-    :type value: np.ndarray
-    :param mean: mean value
-    :type mean: float
-    :param std: standard deviation value
-    :type std: float
-    :return: z-score
-    :rtype: np.ndarray
-    """
-    return (value - mean) / std