Nuke the TinyMCE help dialog as it is duplicated by help tabs. This leaves just Keyboard Shortcuts.

fixes #27100.

Built from https://develop.svn.wordpress.org/trunk@28032


git-svn-id: http://core.svn.wordpress.org/trunk@27862 1a063a9b-81f0-0310-95a4-ce76da25c4cd