TinyMCE: fix `send_to_editor()`. It no longer needs to replace shortcode strings with html placeholers before inserting them in the editor. This is handled properly by the editor's `'BeforeSetContent'` event callbacks. See #24067.

git-svn-id: https://develop.svn.wordpress.org/trunk@26945 602fd350-edb4-49c9-b593-d223f7449a82