Meta: Remove object subtype handling from register_meta().

Registration is now based solely on object type, which allows the code around this to be simplified significantly.

In the process of making this adjustment:

* `register_meta()`, `unregister_meta_key()`, `get_registered_metadata()`, and `registered_meta_key_exists()` no longer return `WP_Error` objects.
* The recently introduced `wp_object_type_exists()` function and the restriction on object type has been removed.

Note: No guarantee of uniqueness is made across object subtypes. Registered meta keys should be uniquely prefixed to avoid conflict.

Fixes #35658.


git-svn-id: https://develop.svn.wordpress.org/trunk@38095 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -5357,31 +5357,6 @@ function mysql_to_rfc3339( $date_string ) {
 	return preg_replace( '/(?:Z|[+-]\d{2}(?::\d{2})?)$/', '', $formatted );
 }
 
-/**
- * Check if an object type exists. By default, these are `post`, `comment`, `user`, and `term`.
- *
- * @since 4.6.0
- *
- * @param  string $object_type Object type to check.
- * @return bool                True if the object type exists, false if not.
- */
-function wp_object_type_exists( $object_type ) {
-	/**
-	 * Filters WordPress object types.
-	 *
-	 * @since 4.6.0
-	 *
-	 * @param array  $types Array of object types.
-	 */
-	$types = apply_filters( 'wp_object_types', array( 'post', 'comment', 'user', 'term' ) );
-
-	if ( in_array( $object_type, $types ) ) {
-		return true;
-	}
-
-	return false;
-}
-
 /**
  * Attempts to raise the PHP memory limit for memory intensive processes.
  *