Eliminate unused `ConfusionMatrix.matrix()` method (#10309)

* fix bug in confusion_matrix

Signed-off-by: Hu Ye <xiaohuzc@gmail.com>

* Update metrics.py

* Update metrics.py

* Update metrics.py

Signed-off-by: Hu Ye <xiaohuzc@gmail.com>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

utils/metrics.py

@@ -177,9 +177,6 @@ class ConfusionMatrix:
                 if not any(m1 == i):
                     self.matrix[dc, self.nc] += 1  # predicted background
 
-    def matrix(self):
-        return self.matrix
-
     def tp_fp(self):
         tp = self.matrix.diagonal()  # true positives
         fp = self.matrix.sum(1) - tp  # false positives