Change default mode cascade in random_walker seg

TODO.txt

@@ -13,8 +13,6 @@ Version 0.11
 Version 0.10
 ------------
 * Remove backwards-compatability of `skimage.measure.regionprops`
-* Change default mode of random_walker segmentation to 'cg_mg' > 'cg' > 'bf',
-  depending on which optional dependencies are available.
 * Remove deprecated `out` parameter of `skimage.morphology.binary_*`
 * Remove deprecated parameter `depth` in `skimage.segmentation.random_walker`
 * Remove deprecated logger function in `skimage/__init__.py`

skimage/segmentation/random_walker_segmentation.py

@@ -344,10 +344,12 @@ def random_walker(data, labels, beta=130, mode='bf', tol=1.e-3, copy=True,
     """
 
     if mode is None:
-        mode = 'bf'
-        warnings.warn("Default mode will change in the next release from 'bf' "
-                      "to 'cg_mg' if pyamg is installed, else to 'cg' if "
-                      "SciPy was built with UMFPACK, or to 'bf' otherwise.")
+        if amg_loaded:
+            mode = 'cg_mg'
+        elif UmfpackContext is not None:
+            mode = 'cg'
+        else:
+            mode = 'bf'
 
     if UmfpackContext is None and mode == 'cg':
         warnings.warn('SciPy was built without UMFPACK. Consider rebuilding '