Varnish: disable WAP on mobile frontends

Unconditionally set X-WAP to "no" on the mobile frontends, essentially
lying to MediaWiki about whether this requests comes from a WAP client.
This entirely disables & kills WAP, per mobile's request. The method of
Keeping X-WAP around gives us the advantage that we can do so without
any MediaWiki changes for now and hence we can quickly rollback the
change, if needed, without even ever poisoning our caches.

After this has been deployed for a while and we're comfortable with it,
we can fix MobileFrontend's mobile detection (using e.g. X-Subdomain or
a separate header) and then kill X-WAP entirely from our VCL.

Change-Id: Ida2afe8b72697d5ba519ac121a2c8ef7ee80aab0