Custom_Image_Header properties:

* rm unused $header_image

* prefer 'public' keyword over deprecated 'var' as per
https://core.trac.wordpress.org/ticket/22234#comment:4
@ehg: do we need this z-index fix?

The concept was introduced in
https://github.com/Automattic/custom-header/commit/1c1da8dafdf4a34fe33be9fbb4a98be7475ba13f
and the index was later bumped because of wpcom's device previews