when using update-alternatives in %postun, the following warnings are emitted:

update-alternatives: warning: alternative /usr/bin/virtualenv-3.6 (part of link
group virtualenv) doesn't exist; removing from list of alternatives
update-alternatives: warning: /etc/alternatives/virtualenv is dangling; it will
be updated with best choice
update-alternatives: using /usr/bin/virtualenv-2.7 to provide
/usr/bin/virtualenv (virtualenv) in auto mode

This is because update-alternatives first checks if it must modify the
auto-selection, and discovers that key files are missing so it auto-corrects
the alternative choice.
...which is what I was telling it to do in the first place.

Unfortunately, using "--quiet" also silences the one non-warning, "using
<something> to provide <something> in auto mode" which seems like a thing we
should keep.

update-alternative should not warn about broken files when instructed to remove
them.