Block Editor: Hide the Custom Fields meta box option if that meta box has been removed.

Some plugins remove the Custom Fields meta box, particularly when they provide functionality that replaces it. The block editor would correctly not display this meta box in these circumstances, but it still showed the option to display or hide it.

Props pento, noisysocks.
Merges [43885] to trunk.
See #45282.
Built from https://develop.svn.wordpress.org/trunk@44244


git-svn-id: https://core.svn.wordpress.org/trunk@44074 1a063a9b-81f0-0310-95a4-ce76da25c4cd