Fix Unsafe Browser vs. APT

Originally created by @intrigeri on #6531 (Redmine)

The Unsafe Browser does not start as:

# apt-get remove  xul-ext-*
Reading package lists... Done
E: The value 'wheezy' is invalid for APT::Default-Release as such a release is not available in the sources

This is presumably due to not shipping APT lists.

Replacing this call with something like that should work:

dpkg -l 'xul-ext-*' | /bin/grep '^ii' | awk '{print $2}' | xargs echo dpkg --remove

Parent Task: #6015 (closed)