Twenty Twelve: fix regression introduced in r23572, allow hide/show of Header Text in Customizer. Fixes #23600.

Also remove another `!important` -- and there was much rejoicing.

Props SergeyBiryukov and obenland.

git-svn-id: https://core.svn.wordpress.org/trunk@23711 1a063a9b-81f0-0310-95a4-ce76da25c4cd