check symlinks can be deleted

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# Date:      Sun Jul 16 16:21:55 2017 +0100
#
# On branch gitish-only
# Changes to be committed:
#	modified:   debian/rules
#	deleted:    orig-symlink
#
