update-alternatives --remove <alternative> foo is
called in the postrm. This can be dangerous because at the time the
postrm is executed foo has already been deleted and update-alternatives
will ignore it while constructing its list of available alternatives.
Then, if the /etc/alternatives symlink points at foo, update-alternatives
won't recognize it and will mark the symlink as something site-specific.
As such, the symlink will no longer be updated automatically and will be
left dangling until
<alternative> is run by hand.
update-alternatives --remove should be called in the prerm