KSES: Conditionally remove the `<form>` element from `$allowedposttags`.

To avoid backwards compatibility issues, `<form>` is re-added if a custom filter has added the `<input>` or `<select>` elements to `$allowedposttags`.

Merges [43994] to trunk.
Built from https://develop.svn.wordpress.org/trunk@44293


git-svn-id: https://core.svn.wordpress.org/trunk@44123 1a063a9b-81f0-0310-95a4-ce76da25c4cd