Drop headless-mixed set; widen asset permissions to all of fileadmin

The headless-mixed set was active although the site runs full headless. On
any request whose Accept header equals exactly 'application/json' it
re-imported the headless page configuration, and 'page < lib.headlessPage'
replaced the whole page object - dropping every field later sets had added:
mainNavigation, footerMenu, metaMenu, megaMenu, jsonLd. The header is not
part of the page cache key, so whichever request rendered first decided what
everyone got, which made it look like a concurrency race. Removing the set
restores the full 15-key payload for every Accept variant.

fix-asset-perms.sh only covered _assets, fileadmin/icons and the extension's
Resources/Public, so anything an importer wrote under public/fileadmin kept
the mount's permissions and returned 403 - most visibly the success-story
images in the megamenu. It now covers public/fileadmin as a whole.

config/sites/vitec/config.yaml

@@ -6,7 +6,6 @@ dependencies:
   - typo3/form
   - typo3/seo-sitemap
   - friendsoftypo3/headless
-  - friendsoftypo3/headless-mixed
   - nb-headless-content-blocks/headless-content-blocks
   - evomedien/vitecset
   - vitec/content-blocks-bundle