DOC remove unnecessary line (scikit-learn#9504)

Author: tobycheese

examples/cluster/plot_cluster_iris.py

@@ -34,7 +34,6 @@
 
 np.random.seed(5)
 
-centers = [[1, 1], [-1, -1], [1, -1]]
 iris = datasets.load_iris()
 X = iris.data
 y = iris.target