Remove test related to deprecated Customizer `export_preview_data()` method.

The method was gutted and deprecated in [36586], so there's no reason to keep
the test.

See #36016.
Built from https://develop.svn.wordpress.org/trunk@36782


git-svn-id: https://core.svn.wordpress.org/trunk@36749 1a063a9b-81f0-0310-95a4-ce76da25c4cd