mlxsw: spectrum_router: Remove outdated comment

Since commit 21151f64 ("mlxsw: Add new FIB entry type for reject
routes") this comment is no longer correct. Remove it.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>