Pretty straightforward:
//tr[not(@id) and not(@class)]
That will give you all tr elements lacking both id and class attributes. If you want all tr elements lacking one of the two, use or instead of and:
//tr[not(@id) or not(@class)]
When attributes and elements are used in this way, if the attribute or element has a value it is treated as if it’s true. If it is missing it is treated as if it’s false.