Don't use disk icon if not generating app dmg icon

[gpoitch]

I don't use the app dmg icon generation with GraphicsMagick, but the base disk-icon.icns is still applied which adds 1.89MB to the dmg. This image is also lost when uploading/downloading.

Can we add an option to disable the icon generation or maybe just skip applying the base icon if GraphicsMagick isn't installed?

[gpoitch]

Here's a command to remove the image after the fact:
xattr -d com.apple.ResourceFork [dmg-file-path]