Revisions: Remove unused post meta data during database upgrade.

Introduced in [23811], WordPress displayed some additional information for restored revisions for a short period of time. This was removed again in [25194], but the meta data was still being saved unnecessarily.

After 3 years and no plugins ever using this meta data, it's time to stop collecting it.

Fixes #36526.

git-svn-id: https://develop.svn.wordpress.org/trunk@37854 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/upgrade.php

@@ -552,6 +552,9 @@ function upgrade_all() {
 	if ( $wp_current_db_version < 36686 )
 		upgrade_450();
 
+	if ( $wp_current_db_version < 37854 )
+		upgrade_460();
+
 	maybe_disable_link_manager();
 
 	maybe_disable_automattic_widgets();
@@ -1687,6 +1690,19 @@ function upgrade_450() {
 	delete_user_setting( 'wplink' );
 }
 
+/**
+ * Executes changes made in WordPress 4.6.0.
+ *
+ * @ignore
+ * @since 4.6.0
+ *
+ * @global int  $wp_current_db_version Current database version.
+ * @global wpdb $wpdb                  WordPress database abstraction object.
+ */
+function upgrade_460() {
+	delete_post_meta_by_key( '_post_restored_from' );
+}
+
 /**
  * Executes network-level upgrade routines.
  *