Remove Spectral Index stuff from attenuate() function in operations_lib.py
The Spectral Index stuff in LSMTool was introduced by @rafferty in a time where WSClean did not yet support correcting apparent sky models to "real" sky models. Therefore there's a part in the apply_beam() function in operations_lib.py that basically tries to determine "apparent spectral indices". This code was neither well-tested, nor is it used often. It was decided to remove the Spectral Index stuff from LSMTool. This MR deals with that.
Closes RAP-704