launcher: init: stop passing --repo-url/--repo-branch down

When the launcher handles the init subcommand, it takes care of
setting the repo url & branch itself when cloning.  So we don't
need to pass them down to the checked out init subcommand.

Further, the init subcommand has never actually done anything
with those options, so there's no point in passing them.

We'll be changing the latter behavior so that init will reset
the url/branch when specified with an existing repo checkout
which means passing them through adds overhead: the launcher
will checkout to the right value, then chain to the sub-init
which will then reset the checkout to the same value.

Bug: https://crbug.com/gerrit/11045
Change-Id: Ia2a4ab9d86febc470aea4abd73d75bb10e848b56
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259312
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>