should delete global is_quiet/is_verbose

[Martin Pool]

In 2.3b1 bzr has a _verbosity_level, is_quiet and is_verbose functions in trace.py. These cause test isolation issues (eg in landing of lp:~mbp/bzr/scripts) and they're not consistently respected. Commands that want a -v or -q option typically want to take some more actions based on that rather than just having trace output suppressed. It would perhaps be perhaps more reasonable to have a verbosity level in the ui object.