d/t/debconf: Fix removing broken symlink
If the path points to a symlink, `pathlib.Path.exists` returns whether
the symlink points to an existing file or directory. So `exists` will
return `False` for broken symlinks. The following `symlink_to` will fail
then.
So just try to remove the symlink and ignore `FileNotFoundError`.
Signed-off-by: Benjamin Drung <benjamin.drung@canonical.com>