Remove old references to imread package

[jarrodmillman]

Description

Checklist

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[stefanv]

@lagru Does it make sense to also deprecate the imread plugin?

[lagru]

I'm not sure about the advantages of imread compared to imageio and other plugins? Looking at it, it seems to be a very thin and simple wrapper, so not a huge loss if we remove it. Trying to load the plugin points the user at http://pypi.python.org/pypi/imread/, so I'd also be alright with keeping it even though it's no longer listed in our dependencies. I'm slightly in favor of deprecating it if there aren't any additional benefits.

[stefanv]

I'm also +0; it's so lightweight that it doesn't really matter either way.