Send the 'wp-remove-post-lock' XHR only when the main window is unloaded...

Send the 'wp-remove-post-lock' XHR only when the main window is unloaded (unloading the Thickbox iframe triggers it too), see #18515

git-svn-id: https://core.svn.wordpress.org/trunk@19258 1a063a9b-81f0-0310-95a4-ce76da25c4cd