On the proof of NP-Hardness of the Cardinality Constrained Quadratic Knapsack Problem
The quadratic knapsack problem is NP-hard.  Therefore, the cardinality-constrained quadratic knapsack problem is NP-hard, too (as Yuval says, just don't use any cardinality constraints).

If problem X is a special case of problem Y, and problem X is NP-hard, then so is problem Y.  Now let X = CCQKP and Y = QKP.