remove unused invocations of np.max

The second arg of np.max is actually an axis, so these
calls to np.max were not clipping the max shifts to be >= 0
(in fact, if you look at tests/utils/test_motion_border.py,
our unit tests were written to expect negative values as a
legal output).

Probably this was not intended, but, all of our code has
been built around the current implementation, so I'm not going
to alter the logic in get_max_correction_values now.

Ticket #458 has been opened to do a more thorough and
self-consistent investigation of our treatment of motion borders
in the future. At that time, we can consider altering
get_max_correction_values to only return values >= 0

src/ophys_etl/utils/motion_border.py

@@ -49,10 +49,10 @@ def get_max_correction_values(x_series: pd.Series, y_series: pd.Series,
     y_no_outliers = y_series[(y_series >= -max_shift)
                              & (y_series <= max_shift)]
     # calculate max border shifts
-    right_shift = np.max(-1 * x_no_outliers.min(), 0)
-    left_shift = np.max(x_no_outliers.max(), 0)
-    down_shift = np.max(-1 * y_no_outliers.min(), 0)
-    up_shift = np.max(y_no_outliers.max(), 0)
+    right_shift = -1 * x_no_outliers.min()
+    left_shift = x_no_outliers.max()
+    down_shift = -1 * y_no_outliers.min()
+    up_shift = y_no_outliers.max()
 
     max_shift = MaxFrameShift(left=left_shift, right=right_shift,
                               up=up_shift, down=down_shift)