First let us note that GW are ripples in space, and are created due to motion of black holes. 

Therefore it should be easy to see that the waves come only from kinetic energy, which in turns comes from converting potential energy of the system into motion of the system.

Change in mass may be taking place due to other reasons/mechanisms that are not relevant to this question. Because, for "change in mass" to contribute to GW, it first have to convert to kinetic energy (of the two BH), and it is difficult to explain, how that would happen. And even if it does by some magic, would it have any noticeable contribution towards GW.

GW are ripples in space and they can only be created by kinetic energy - i.e. by movements or probably by explosions too.

If conversion of mass into energy was creating GW, then all the stars would generate GW (however weak) all the time.