Useless confusing lines for iceweasel in APT preferences

The following lines in /etc/apt/preferences are confusing as:

- they won’t actually block the installation of iceweasel which is still in debian repos
- iceweasel is now removed from our apt repo

+Explanation: Block installation of iceweasel until it has been removed from our APT repo
+Package: iceweasel
+Pin: origin deb.tails.boum.org
+Pin-Priority: -1

See also discussion at https://mailman.boum.org/pipermail/tails-dev/2014-September/006944.html

Feature Branch: bugfix/8038-remove-useless-iceweasel-apt-pinning

Parent Task: #7953 (closed)

Original created by @alant on 8038 (Redmine)