Customizer: Avoid hiding 'Remove' buttons unrelated to custom headers.

Merges [28266] to the 3.9 branch.

props rzen, ehg.
fixes #27848.


git-svn-id: https://develop.svn.wordpress.org/branches/3.9@28267 602fd350-edb4-49c9-b593-d223f7449a82