Remove reviewer setting, let autoassign take over (#2094)

Fixes #1841

The current autoassign doesn't work, but I'm trying to fix it (e.g., #2093)

proposals/scripts/new_proposal.py

@@ -198,8 +198,6 @@ def main() -> None:
             "proposal",
             "--label",
             "proposal draft",
-            "--reviewer",
-            "carbon-language/carbon-leads",
             "--repo",
             "carbon-language/carbon-lang",
             "--title",