[OVN] Stop doing PgDelPortCommand on each router port update

[Maciej Jozefczyk]

I noticed while we do a router port update, on each call there is a command:

Jul 31 08:17:15 devstack-mjozefcz-ovn-octavia-provider neutron-server[26580]: DEBUG ovsdbapp.backend.ovs_idl.transaction [None req-61de5786-9d32-4d52-8a24-56af3d400ce2 None None] Running txn n=1 command(idx=2): PgDelPortCommand(port_group=neutron_pg_drop, lsp=['7bf20b51-8b40-448e-a528-54d9727dbddc'], if_exists=False) {{(pid=26587) do_commit /usr/local/lib/python3.6/dist-packages/ovsdbapp/backend/ovs_idl/transaction.py:87}}

It is not needed on each call, because:
1) only if a router port is created we shouldn't add it to default pg_drop (we don't filter traffic on router ports)
2) only if a normal port is being bound to a router we should drop this default pg_drop

In all other cases - it is not needed. We will save CPU cycles.