iconcache: Remove want_fallback hint

It's unused, and if it was ever set, it would cause an assert fail.
iconcache: Remove commented out code
iconcache: Remove dead code
iconcache: Group declarations where they're used
core: Remove useless meta_invalidate_default_icons implementation

This code was supposed to refresh our default icons when the theme
changed, but it actually was a no-op, since the default icons are cached
in a static variable in MetaUI.

I'm not sure the fact that the fallback icons don't update when the
theme changes is an important enough use case to keep working, but I'm
keeping the skeleton function there in case somebody wants to actually
fix it properly.
iconcache: Remove a useless icon_cache_free

All it does is poke a few fields. There is no point to it.
window: Move X11-specific icon code to MetaWindowX11