Remove apt::pin for squid packages

In the pre-Varnish era, Wikimedia production was based on squid 2.x.
When we did the precise upgrade, squid 3.x came, so we had to include
our own builds of squid 2.x package in our repository.

install-server & url_downloader were transitioned to squid 3.x long ago
and for that they needed apt pins to use Ubuntu's version rather than
our own.

Now that we don't need this anymore and those packages have been removed
from apt, remove our custom apt::pin.

Note that this doesn't actually remove the pin, as apt preferences.d is
not puppet-managed; since it's just two hosts, it will cleaned up
manually after this gets merged.

Change-Id: Ibc44681745c44e4ec6dafdf40dc8838032327a9e