Stop using .rtl classes, instead relying on flipping LTR CSS to RTL.

props yoavf.
fixes #24977.

Built from https://develop.svn.wordpress.org/trunk@26243


git-svn-id: http://core.svn.wordpress.org/trunk@26150 1a063a9b-81f0-0310-95a4-ce76da25c4cd