Backgrounds: Do not show our default wallpaper as duplicate

The default wallpaper is overlayed in vendor/lineage, currently
based on ascension_teal_dark. Remove it from the partner_wallpapers
array so that it doesn't appear twice when the user navigates to
"On-device wallpapers", as default one is always shown in the
first position.

Leave the image still here as placeholder since it's the original
and served for running update_default_wallpaper.sh script.

Change-Id: I8f5b68f7f2c30438f8248c46c1cf807ade345ae5