Issue #2015789 by fubhy, echoz, rteijeiro, LewisNyman, penyaskito: Remove language_css_alter() (RTL stylesheets) in favor of HTML 'dir' attribute.

Replacement PR completing https://github.com/backdrop/backdrop/pull/305

Backdrop issue: https://github.com/backdrop/backdrop-issues/issues/207
Drupal.org issue: http://drupal.org/node/2015789