Remove custom dtype range

Bool support was added in gh-#260

skimage/viewer/plugins/overlayplugin.py

@@ -1,15 +1,8 @@
-import numpy as np
-
-from skimage.util import dtype
+from skimage.util.dtype import dtype_range
 from .base import Plugin
 from ..utils import ClearColormap
 
 
-#TODO: Maybe this bool definition should be moved to skimage.util.dtype.
-dtype_range = dtype.dtype_range.copy()
-dtype_range[np.bool_] = (False, True)
-
-
 class OverlayPlugin(Plugin):
     """Plugin for ImageViewer that displays an overlay on top of main image.