repo: stop using our repo fork

We aren't using --git-cache on bots anymore, so we shouldn't need to
point people to the CrOS fork.  Everything else is merged upstream.

Bug: b/187784544
Change-Id: Ifd261ce509b9b501c49c2b567be17e4d9014913c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5311255
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: George Engelbrecht <engeg@google.com>

repo

@@ -50,9 +50,6 @@ def main(argv):
     if argv and argv[0] == 'sync':
         _UpdateDepotTools()
 
-    # Set the default to our fork.
-    os.environ["REPO_URL"] = "https://chromium.googlesource.com/external/repo"
-
     os.execv(sys.executable, [sys.executable, str(REPO)] + argv)