ENH: Remove unused copy in k-means

One copy was done above the copy removed.

doc/whats_new.rst

@@ -6,6 +6,12 @@
 ======
 
 
+Enhancements
+------------
+
+   - Minor performance improvements to k-means by `Gael Varoquaux`_.
+
+
 Bug fixes
 ---------
 

sklearn/cluster/k_means_.py

@@ -237,8 +237,7 @@ def k_means(X, n_clusters, init='k-means++', precompute_distances=True,
     if not sp.issparse(X) or hasattr(init, '__array__'):
         X_mean = X.mean(axis=0)
     if not sp.issparse(X):
-        if copy_x:
-            X = X.copy()
+        # The copy was already done above
         X -= X_mean
 
     if hasattr(init, '__array__'):