varnish: remove X-CS/Analytics tag from vcl_fetch

Do not tag with X-CS/X-Analytics on vcl_fetch, as this would be injected
in cache even in non-varied responses & hence served out of cache to
non-zero users as well.

Move this to vcl_deliver and while at it:
- remove the X-CS tagging completely -- it shouldn't be needed(?)
- set (or unset) X-Analytics unconditionally. This effectively means
  that if MediaWiki sets it, it *will* be overriden; however, this is
  the only way to retroactively fix the problem, now that the cache is
  poisoned with incorrectly tagged cached objects.

Bug: 53806
Change-Id: I6ac2dde5d1a81874db60e69075090becfbe04eaa