Defuse a ticking time-bomb by removing the dependency on devel/bzr.

It is likely that this will break "make test", so, why am I doing it?

The answer is that devel/bzr unconditionally depends on python27 (it is
known not to build with python37) -- and python27 is imminently due for
removal (see PR 249337).

This would not matter so much except that py-pip winds up a dependency
of devel/cmake, and thus, over 2000 ports -- all of which would have
broken when python27 was removed.

Unfortunately the time before that removal happens is simply too short
to have a proper discussion with all stakeholders. We need to remove
the dependency immediately so that PR 249337 can proceed.

PR: 249337 (related to)