`shortage_max_timestep` is not implemented

[smartie2076]

Check if shortage_max_timestep (variable: SHORTAGE_MAX_TIMESTEP) is implemented correctly, as it could also be used to define the share of non-critical loads.

SHORTAGE_MAX_TIMESTEP should be introduced as constraint in: https://github.com/rl-institut/offgridders/blob/dev/src/G2b_constraints_custom.py#L959

Function is never called in Offgridders!