Bug#1120935: debian-policy: Remove /usr/share/common-licenses/BSD reference

All references that are in copyright files or part of binary packages are gone
now. There are some remaining in upstream source but this is not
regulated by the Policy.

The only packages left with references are linters that warn users about
tis file reference.

So in my opinion we can move forward with a "must" in the Policy and
when that is released, remove the file from base-files.