Block Editor: Prevent the meta box form from being accidentally submitted.

Meta boxes are loaded inside a wrapper form, that the block editor submits as part of the saving process. It shouldn't be submitted manually.

Props pento.
Merges [43880] to trunk.
Fixes #45284.
Built from https://develop.svn.wordpress.org/trunk@44240


git-svn-id: https://core.svn.wordpress.org/trunk@44070 1a063a9b-81f0-0310-95a4-ce76da25c4cd