Remove the shadow on the TinyMCE editor.

This removes it from distraction-free writing and also gives the editor a bit of a lighter feel.

fixes #26421.

Built from https://develop.svn.wordpress.org/trunk@26659


git-svn-id: http://core.svn.wordpress.org/trunk@26549 1a063a9b-81f0-0310-95a4-ce76da25c4cd