Remove obsolete occurence of X-ApertisManifestUrl

As per T3494, this field is obsolete.

Apertis: https://phabricator.apertis.org/T2702



Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.com>
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D6138