Make Debian package working on Ubuntu again.

Make Debian package working on Ubuntu again.

Debian supports symbolic links to xulrunner-stub which eases packaging
a lot. Copying or hardlinking files from other packages is just dumb.

But Ubuntu ships xulrunner without this feature so the previous
debian/conkeror.bin didn't work on Ubuntu.

This commit effectively reverts the commits
db67ee42bfab73c1699a0303717fdc93e0731f01,
6475fd23bcd85d598b2f7b1a6800515c73b131c8 and
f3e019855871300c1799f47d1174df03be48456e.

Mike Hommey, I'm sorry to abandon the work you did to help me making
use of the symlinking feature.

Alexander Sack, thanks for confirming that Ubuntu doesn't have the
symlinking feature.