TinyMCE: don't import color and background-color from the content when creating menus that have preview capabilities.

Fixes a visual glitch: when post formats styling is added in editor-styles.css, it is applied to the menus and not reset after changing format. See #24308.

git-svn-id: https://core.svn.wordpress.org/trunk@24341 1a063a9b-81f0-0310-95a4-ce76da25c4cd