Docs: Remove @global tag for $wpdb from wp-admin/install.php and wp-admin/upgrade.php.

Per the documentation standards, the `@global` tag is meant to list PHP globals used within functions or methods.

The code in question uses the variable in global namespace, but does not explicitly declare it.

Props jayupadhyay01, dswebsme.
Fixes #46602.

git-svn-id: https://develop.svn.wordpress.org/trunk@45233 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/install.php

@@ -229,7 +229,6 @@ if ( is_blog_installed() ) {
  * @global string $wp_version
  * @global string $required_php_version
  * @global string $required_mysql_version
- * @global wpdb   $wpdb
  */
 global $wp_version, $required_php_version, $required_mysql_version;