gtk3.setupHook: clear icon-theme.cache in preFixup

Packages often run gtk-update-icon-cache to include their icons in themes’ icon cache. However, since each package is installed to its own prefix, the files will only collide. For that reason we are removing the icon-theme.cache from applications.

Previously, we did that in hicolor-icon-theme setup hook but, since it is actually gtk3’s utility that creates the cache, we thought it would be appropriate to let its setup hook handle the clearing.