Remove comment about cmap from voxels docstring

voxels does not use cmap at all, and thus does not modify shade in relation to cmap being passed in, this appears to be copy/pasted from methods which _do_ use cmap

Author: ksunden

lib/mpl_toolkits/mplot3d/axes3d.py

@@ -2801,8 +2801,7 @@ def voxels(self, *args, facecolors=None, edgecolors=None, shade=True,
               last axis.
 
         shade : bool, default: True
-            Whether to shade the facecolors.  Shading is always disabled when
-            *cmap* is specified.
+            Whether to shade the facecolors.
 
         lightsource : `~matplotlib.colors.LightSource`
             The lightsource to use when *shade* is True.