TinyMCE: fix `send_to_editor()`. It no longer needs to replace shortcode...

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.
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git-svn-id: https://core.svn.wordpress.org/trunk@26824 1a063a9b-81f0-0310-95a4-ce76da25c4cd