Accessibility: remove title attributes from the available headers in the (no...

Accessibility: remove title attributes from the available headers in the (no more used) `custom-header.php` screen.

Fixes #35062.
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git-svn-id: https://core.svn.wordpress.org/trunk@35869 1a063a9b-81f0-0310-95a4-ce76da25c4cd