Just edit the start g-code if you only want one rear purge line add ; to disable the second purge line like this and save it.
M109 S{nozzle_temperature_initial_layer[initial_no_support_extruder]}
G0 X253 E6.4 F{outer_wall_volumetric_speed/(0.3*0.6) * 60}
G0 Y128 E6.4
;G0 X252.5
;G0 Y252.5 E6.4
;G0 X128 E6.4