Customizer: Re-add the possibility to remove and restore a default background image.

see #21483.
fixes #30683 for trunk.
Built from https://develop.svn.wordpress.org/trunk@30849


git-svn-id: http://core.svn.wordpress.org/trunk@30839 1a063a9b-81f0-0310-95a4-ce76da25c4cd