Also remove `enum` for validating allowed templates to allow plugins to dynamically supply their own templates for specific posts, even when they are not in the theme.
Props TimothyBlynJacobs, jnylen0, swissspidy.
Fixes#39996.
Built from https://develop.svn.wordpress.org/trunk@41979
git-svn-id: http://core.svn.wordpress.org/trunk@41813 1a063a9b-81f0-0310-95a4-ce76da25c4cd