Twenty Fifteen: admin-custom-header.css was emptied in r30567 so let's remove it entirely.

Props kwight, fixes #30648.


Built from https://develop.svn.wordpress.org/trunk@30801


git-svn-id: http://core.svn.wordpress.org/trunk@30791 1a063a9b-81f0-0310-95a4-ce76da25c4cd