SmallTable can store the same rule more than once

[nding0405]

1. Problem
   The comment says that an item must not be added twice:

/**
 * Inserts a new element into the table.  Does nothing if the element is
 * already present.
 */

However, the check is commented out:

// if (contains(element))
{
	// return;
}

[SmallTable::insert()](https://github.com/CHERIoT-Platform/network-stack/blob/58425e6a1c65c4357b037aa817ba492e7a8fa70b/lib/firewall/firewall.cc#L238-L257)