glocalfileinfo: don't special-case directory icons

As of e6af432, g_content_type_get_symbolic_icon() returns non-symbolic
fallbacks. Thus, we can't append another symbolic icon to the fallbacks.

The special case was a bit of a hack anyway. It was only applied to
themed icons and there was no generic fallback for mime types that are
not folders.

https://bugzilla.gnome.org/show_bug.cgi?id=726046