Twenty Fifteen: Remove custom header admin callbacks.

[philiparthurmoore]

In [30459] 4.1 sends users directly to the Customizer when they want to modify their custom header. Is it okay to ditch all the twentyfifteen_admin_header_image stuff yet? If so, patch attached.

[celloexpressions]

+1. Users will only be able to access the old pages in the following circumstances now (mostly cases where Customizer isn't supported):

• No JS
• Using IE7
• Using IE8 or IE9 on multisite with domain mapping
• If the user has a bookmark to this screen or has a direct link here from anywhere outside core.

These cases simply aren't worth all of the complexity that this code adds to the theme from a developers' perspective. Additionally, given the sidebar/header setup in Twenty Fifteen, even with all of this code, the header "preview" is still very confusing from a UI perspective.

[iamtakashi]

Lots of red in the patch. I like it :)

[iandstewart]

In 30567:

Twenty Fifteen: remove unneeded custom header admin callbacks.

Props philiparthurmoore, fixes #30432.