just as most scripts are skipped if their stagefile exists (indicating
that they have already been run to completion), including chroot
preparation scripts in install mode, this implements the same guard for
chroot prep remove mode, such that they exit early if their stagefile
does not exist, indicating that the modification has already been removed.
(also override-able by --force in the same way).
this basically just uses a tweaked copy of Check_stagefile().
Gbp-Dch: Short
d79c96232b