Fixed an issue where hundreds of Warning are displayed while define('WP_DEBUG', true); is present; more details: "first argument of wpdb::prepare should have a placeholder".

A checked was performed to find all files containing $wpdb->prepare, using
- grep -ri 'wpdb->prepare' *

And the result showed:
- plugins/wordpress-https/lib/WordPressHTTPS.php:
- $multisite_hosts = $wpdb->get_col($wpdb->prepare("SELECT domain FROM " . $wpdb->blogs, NULL));

- plugins/wordpress-https/lib/WordPressHTTPS.php:
- $blogs = $wpdb->get_col($wpdb->prepare("SELECT blog_id FROM " . $wpdb->blogs, NULL));

- plugins/wordpress-https/uninstall.php:
- $blogs = $wpdb->get_col($wpdb->prepare("SELECT blog_id FROM " . $wpdb->blogs, NULL));

Replaced 3 instances of where $wpdb->prepare() second argument was passed as null and possibly opens up a vulnerability.

Summary of the change:
- Orignial: $wpdb->get_col($wpdb->prepare("SELECT domain FROM " . $wpdb->blogs, NULL));
- New:      $wpdb->get_col($wpdb->prepare("SELECT domain FROM %d", $wpdb->blogs));

For information see the ticket that introduced the notice in WP3.9 https://core.trac.wordpress.org/ticket/25604