Drop custom icons in gcr

Icons are problematic to provide within gcr, as different DEs that are using it, have different expectations what such an icon should look like: one wants it to be colorful, another wants it to be monochrome, yet another wants everything skeumorphic etc.

Let's not deal with this in gcr4 and drop any icons. If applications want to provide an icon, they can do that themselves much better then we ever can.