varnish: remove default Content-Type coercion

There were two reasons we thought we might need to preserve this behavior:

(a) We were worried that backend responses without a content-type were
    prevalent (but masked by DefaultType), and that removing the default
    content-type header would break things in ways that were hard to anticipate.
(b) We were worried that an attacker could somehow craft requests that trigger
    responses that compromise users' browsers.

Ten days' worth of logs indicate that responses without a default type occur
very rarely. (In fact, we've only seen them for <https://doc.wikimedia.org/>,
which was not previously setting a default type at all.) So (a) is no longer
a credible worry.

Where does that leave (b)? If the threat is immaterial, it'd be good to remove
this hack and conform to standards.

The backend responses that were logged as not having a content-type are listed
here: <http://noc.wikimedia.org/~ori/defaulttype_resps.log>
(The ommission of the host name is accidental, but it can be determined from
the path that they are all for doc.wikimedia.org.)

Change-Id: If0c4fc8fd0eedb0df01949fda9a420bddfbb0068