Remove drop shadow from app icons

We’ve previously applied a “shelf” style shadow to app icons that fits in with a shelf style dock. However, we no longer have this style in the dock.

Additionally other freedesktop platforms like GNOME are recommending app authors not to apply drop shadow to their app icons at all, and they handle app icon shadows on the shell so that they’re applied consistently and scaled uniformly. This means that many third party icons now have no drop shadows and look out of place in Pantheon

We should probably go ahead and remove drop shadows from our app icons as well and do shadows in the shell so that we don’t end up with a mix of app icons with shadows and some without