Customize: Remove incorrect test assertions for the `post_date` of customization drafts.

Amends [41887].
See #42220.

Built from https://develop.svn.wordpress.org/trunk@41892


git-svn-id: https://core.svn.wordpress.org/trunk@41726 1a063a9b-81f0-0310-95a4-ce76da25c4cd