remove unused div with class wp-menu-arrow

[jbkkd]

Upon starting a fix on bug this bug: #29121

I noticed a div in the wp-admin with class wp-menu-arrow, which is never used.
This patch removes all reference to that div and class.

Not sure if I need to include the minified versions as well or if it's done automatically, let me know if it's needed.

[helen]

Thanks for the patch! This is a duplicate of #26960 - feel free to add your patch over there (or just link to this one) so we can reference it :)