Updated Manager._remove() prompt polarity.

OLD: prompt=False.
NEW: prompt=True.

MESSAGING: messaging keywords should all be set to true by default.

Messaging keywords were previously limited to prompt=True. In future commits
prompt=True will be replaced by the combination of confirm=True and
message=True. (Methods that ask for confirmation from the composer will
implement a confirm=True keyword; methods that display output will implement a
message=True keyword.)