Always return Precision metric regardless of `plot` arg (#16900)

ultralytics/utils/metrics.py

@@ -598,7 +598,7 @@ def ap_per_class(
         # AP from recall-precision curve
         for j in range(tp.shape[1]):
             ap[ci, j], mpre, mrec = compute_ap(recall[:, j], precision[:, j])
-            if plot and j == 0:
+            if j == 0:
                 prec_values.append(np.interp(x, mrec, mpre))  # precision at mAP@0.5
 
     prec_values = np.array(prec_values)  # (nc, 1000)