Comment 10 for bug 1592870

My initial thought is that ubuntu-wallpapers.xml.in should have
ubuntu-default-greyscale-wallpaper.png

and we should remove Yak_Wallpaper_Grey_4096x2304.png from yakkety-wallpapers.xml.in
and remove Suru_Wallpaper_Desktop_4096x2304_Gray.png from ubuntu-wallpapers.xml.in

That way, users who use the Settings app to set their wallpaper to the grey version in 16.10 will automatically get the new gray version when they upgrade to 17.04.

I believe we've previously always removed the old aubergine wallpapers with new versions (Ubuntu doesn't make it easy for someone to keep the 14.04 version of the default wallpaper when upgrading to 16.04. On the other hand, 10 slightly different aubergine wallpapers in the chooser for someone who upgrades to every Ubuntu release is probably a problem.). If we're going to change that now for the gray version then we should change it for the aubergine version too.