Remove viridis

[dopplershift]

So now that our minimum matplotlib is guaranteed to have the "viridis" colormap, we should remove our own copy of it. Now...is this worth deprecating? If so we'd have to:

1. Add a custom __getitem__ to ColortableRegistry since it inherits from dict
2. Look for 'viridis' and issue a MetpyDeprecationWarning if someone asks for it.

I'm tempted to avoid that work and just remove it. Either way, the NEXRAD_Level_2_File.py will need updating to just pull it from matplotlib.

[zbruick]

Could we somehow add an error message saying to import it from matplotlib if a user calls it (i.e. if we rip it out, just add a custom ImportError message)? Or to do any sort of warning/error, we have to go through this work? It does seem like a decent amount of work...I don't know how often people are using this still, if giving a migration path is worth the work.

[dopplershift]

Unfortunately, there is no way to get any message without hooking into __getitem__. It's not a ton of work, just a dozen or so lines to add just to get an error out. If we think our users need it, then they need it, and we can add. It might be the nice thing to do regardless.