Omitting unnecessary constraints on the CHP units

[leonsn]

All CHPs in pypsa-eur-sec are operating on the backpressure line and this set of extra constraints to ensure working in the operational space of the CHPs is hence not necessary at the moment.

[fneum]

I don't think the add_chp_constraints() takes any effect currently already because both heat_bool and electric_bool will always be empty with @nworbmot's changes in 47a618f

I'm fine with removing the function call entirely in the extra_functionality and keeping the function itself in case we want to use it some later time.