Delete git-auto-svn and its docs

We no longer support git-svn as a workflow.

R=iannucci@chromium.org
BUG=475320

Review-Url: https://codereview.chromium.org/2360143002
diff --git a/git_cl.py b/git_cl.py
index 1b016d4..dbf0d2c 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -4400,8 +4400,8 @@
   if not settings.GetIsGitSvn():
     if git_footers.get_footer_svn_id():
       # If it looks like previous commits were mirrored with git-svn.
-      message = """This repository appears to be a git-svn mirror, but no
-upstream SVN master is set. You probably need to run 'git auto-svn' once."""
+      message = """This repository appears to be a git-svn mirror, but we
+don't support git-svn mirrors anymore."""
     else:
       message = """This doesn't appear to be an SVN repository.
 If your project has a true, writeable git repository, you probably want to run