REF: Remove image change percentage function

Remove image change percentage function from the `analysis.utils` module as it is a metric of image similarity rather than a metric of motion.

Co-authored-by: Oscar Esteban <code@oscaresteban.es>

Author: jhlegarreta

nitransforms/analysis/utils.py

@@ -95,39 +95,6 @@ def compute_fd_from_transform(
     return np.mean(np.linalg.norm(test_xfm.map(fd_coords) - fd_coords, axis=-1))
 
 
-def compute_percentage_change(
-    reference: np.ndarray,
-    test: np.ndarray,
-    mask: np.ndarray,
-) -> np.ndarray:
-    """Compute motion change between reference and test as a percentage.
-
-    If a mask is provided, the computation is only provided within the mask.
-    Also, null values are ignored.
-
-    Parameters
-    ----------
-    reference : :obj:`~numpy.ndarray`
-        Reference imaging volume.
-    test : :obj:`~numpy.ndarray`
-        Test (shifted) imaging volume.
-    mask : :obj:`~numpy.ndarray`
-        Mask for value consideration.
-
-    Returns
-    -------
-    rel_diff : :obj:`~numpy.ndarray`
-        Motion change between reference and test.
-    """
-
-    # Avoid divide-by-zero errors
-    eps = 1e-5
-    rel_diff = np.zeros_like(reference)
-    mask = mask.copy()
-    mask[reference <= eps] = False
-    rel_diff[mask] = 100 * (test[mask] - reference[mask]) / reference[mask]
-
-    return rel_diff
 
 
 def displacements_within_mask(