Dgit.pm: Abolish now-unused $t_local_git_cfg playtree_setup arg

This was there so that dgit's -c options would be honoured here.
But now, we get this information via @git, which dgit adds -c
options to, which we pass to git-playtree-create by invoking it
via git.

So this is no longer used and can be abolished.

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