url_downloader: squid3 cache_dir fix

In squid 2.6, 2.7 and 3.0 disabling the on disk cache required the usage
of a cache_dir null declaration using the null storage type. In 3.1 and
upwards this is now accomplished by not declaring any cache_dir
directives at all. As a result the null type was also dropped and the
specified directive in the configuration prevents squid from starting.
However, in 3.3 (at least in Ubuntu land), a cache_dir null directive no
longer stops the server from starting but is rather silently ignored.
This seems to be an un-documented behavior, at least in:
http://www.squid-cache.org/Doc/config/cache_dir/ or
http://bazaar.launchpad.net/~squid/squid/trunk/view/head:/ChangeLog and
a quick traversing of the code did not turn up anything. Fix the problem
on 3.1 (precise) by omitting the cache_dir directive
Please do note the in memory caching has nothing to do with any of this.

Change-Id: Ie623bb0fdce584e578e2f440b1ab213b64a99fdd