Revert "texture-cache: Use image-missing image when no other icon was loaded"

Revert "texture-cache: Use image-missing image when no other icon was loaded"

The commit broke StIcon's :fallback-gicon property, as it relies on failure to load
an icon to determine that the fallback should be shown.

Luckily StIcon is the only user of st_texture_cache_load_gicon() (at least in
regular shell code), so we'll be able to implement the 'image-missing' fallback
there.

This reverts commit 3dd8ffc2bb1cefd603bcadabd8886dda3192dbd5.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1127