Remove folder gradient

This merge request's goal is to remove the green folder gradient that was added in a recent commit. The gradient used a green-blue pattern. The problem with this is that green is used nowhere in the Adwaita stylesheet or icon set (except in a few edge cases [like the trash icon]), so it makes the folders look completely out of place. Adwaita is mainly a blue theme, so the folders should be blue as well. The folders should match the rest of the system. Frankly, green does not match the rest of the system.

Also, if GNOME ever adds a color preference in the settings, it's not going to make sense that the folders would be a gradient instead of a simple solid color. It is far more logical for the folders to be a single color in a technical and design sense.