Remove null plugin since we have the pil plugin as a requirement

skimage/io/_plugins/null_plugin.ini

@@ -1,3 +0,0 @@
-[null]
-description = Default plugin that does nothing
-provides = imshow, imread, imsave, _app_show

skimage/io/_plugins/null_plugin.py

@@ -1,25 +0,0 @@
-__all__ = ['imshow', 'imread', 'imsave', '_app_show']
-
-import warnings
-
-message = '''\
-No plugin has been loaded.  Please refer to the docstring for ``skimage.io``
-for a list of available plugins.  You may specify a plugin explicitly as
-an argument to ``imread``, e.g. ``imread("image.jpg", plugin='pil')``.
-
-'''
-
-
-def imshow(*args, **kwargs):
-    warnings.warn(RuntimeWarning(message))
-
-
-def imread(*args, **kwargs):
-    warnings.warn(RuntimeWarning(message))
-
-
-def imsave(*args, **kwargs):
-    warnings.warn(RuntimeWarning(message))
-
-
-_app_show = imshow