You asked how to remove it, so I assumed that you would want to do it properly. The wiring most likely will go to the wiring centre, but that would depend upon the personal preferences of the original installer.
If you wanted to, you can leave the Heatmiser set to be 'on' permanently and at maximum temperature (I assume it is a PRT model with time and temperature control) and that will allow the Hive to take overall control. Alternatively if you are happy to leave the Heatmiser in place, you could simply move the brown wire from 'L' and put it in 'A2' with the green / yellow and black tape wire. (remove the brown link too) That way the Heatmiser won't be consuming any power and unlike the leaving it permanently 'on' can't be tampered with, or cause a problem for the Hive in the future if it develops a fault.