Remove spl_types from the generated manual

[TysonAndre]

Related to https://github.com/php/doc-en/pull/448

Other unmaintained extensions have also been removed, e.g. dc5f3f12024b6af0374e912253991660d7438480

• https://pecl.php.net/package/SPL_Types was last updated in 2012 and is not maintained.
• SPL_Types is easy to confuse with the SPL because the same prefix is used
• SPL_Types can cause confusion because PHP has typed properties(7.4), and will probably also have actual enums in 8.1 https://www.php.net/spl_types

[cmb69]

Obsolete due to 9c830ba83edd0bad872237cee414b31588219c35.