[HAT] Remove DefaultType directive from Apache config

<http://httpd.apache.org/docs/2.4/upgrading.html>:
"The DefaultType directive no longer has any effect, other than to emit a
warning if it's used with any value other than none."

As of Ia36d0d89f, this is handled by Varnish.

RT: 7700
Change-Id: Icbeb0ce369526bda401d449e83383f2baad4d159