Deprecate rich_edit_exists(). It doesn't make sense to support deleting the...

Deprecate rich_edit_exists(). It doesn't make sense to support deleting the TinyMCE directory when we have auto-updates. Fixes #26786.
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git-svn-id: https://core.svn.wordpress.org/trunk@26814 1a063a9b-81f0-0310-95a4-ce76da25c4cd