REST API: Remove specific multi-type schema handling from the themes controller.

Multi-type schema handling was improved in [48306]. In particular, it now allows for sanitizing a multi-typed value that wouldn't validate. Removing this handling will make 3rd party registered theme features more robust.

Fixes #50562.

Built from https://develop.svn.wordpress.org/trunk@48308


git-svn-id: https://core.svn.wordpress.org/trunk@48077 1a063a9b-81f0-0310-95a4-ce76da25c4cd