TinyMCE: fix editor.css.
- Re-enable the default images for buttons not used in WordPress.
- Remove redundant styles for the old buttons sprite.
Fixes #26294.
Built from https://develop.svn.wordpress.org/trunk@26446


git-svn-id: http://core.svn.wordpress.org/trunk@26344 1a063a9b-81f0-0310-95a4-ce76da25c4cd