Long ago (before version 0.99) we used to build a tree of symlinks in
$DESTDIR rather than copying files, then use the cpio --dereference
option.  This made broken symlinks fatal, so we have code to delete
them before running cpio.

We no longer use symlinks that way, and symlinks under $DESTDIR
might appear broken even though they will be valid at boot time.  In
particular, vmdebootstrap installs packages in a chroot without /proc
mounted, thus ${DESTDIR}/etc/mtab is a broken link and this command
deletes it.  That stops fsck from running in the initramfs, or at
all (in most cases) until something re-runs update-initramfs.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Closes: #845581