Multisite: Remove the redundant blog_versions table.

As part of the Multisite installation process, a `blog_versions` table is created. This table is never read from (except immediately prior to updating it), it's only ever inserted into or updated. It is not used to determine which blogs need to be upgraded.
This table was introduced in 3.0 when the WPMU schema was merged #11644 and it appears the table has never been used in core and is therefore redundant and may as well be removed.

Props johnbillion, nacin, ryan, johnjamesjacoby, whyisjake.

Fixes #19755. See #41685.



git-svn-id: https://develop.svn.wordpress.org/trunk@46194 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/wp-db.php

@@ -306,7 +306,6 @@ class wpdb {
 		'sitemeta',
 		'sitecategories',
 		'registration_log',
-		'blog_versions',
 	);
 
 	/**
@@ -425,14 +424,6 @@ class wpdb {
 	 */
 	public $blogmeta;
 
-	/**
-	 * Multisite Blog Versions table
-	 *
-	 * @since 3.0.0
-	 * @var string
-	 */
-	public $blog_versions;
-
 	/**
 	 * Multisite Registration Log table
 	 *