Twenty Fifteen: remove Genericons as a dependency from loading the main stylesheet.

Props iamtakashi, fixes #30532.
Built from https://develop.svn.wordpress.org/trunk@30865


git-svn-id: http://core.svn.wordpress.org/trunk@30855 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fourteen: remove Genericons as a dependency from loading the main stylesheet and IE-specific stylesheet. See #30532.
Built from https://develop.svn.wordpress.org/trunk@30866


git-svn-id: http://core.svn.wordpress.org/trunk@30856 1a063a9b-81f0-0310-95a4-ce76da25c4cd