varnish mobile: don't override MW's X-Analytics

Commit I6ac2dd altered the config to override previously set & cached
X-Analytics. This unfortunately included X-Analytics as set by
MediaWiki, which was flagged by the commit message but considered to be
okay at the time.

Turns out it wasn't, so let's be a bit more smart about it and only
unset the previously set X-Analytics only when the request isn't varied
by X-CS. This should have the same effect but without its downsides.

Bug: 54779
Change-Id: Iaf6a7255a5dffd32e43233b19fbeffd60de50cc4