MAgPIE coupling: removed s56_ghgprice_start from coupling script, sin…

Created by: dklein-pik

…ce the start year of the CO2 price is solely defined by the data transfered from REMIND. The parameter was once introduced to define the beginning of the CO2 price phase-in in MAgPIE. Since the phase-in is now done in REMIND and no longer in MAgPIE, the parameter is no longer needed in the coupling.