remove or fix desitarget.lyazcat

[sbailey]

It appears that the functionality of desitarget.lyazcat has been replaced by the desispec creation of zmtl and qso_qn afterburner files (desispec.zmtl and desispec.scripts.qsoqn). If it really is unneeded, remove desitarget.lyazcat.

If we do really still need desitarget.lyazcat, it will need to be updated to support the quasarnp API changes in desihub/QuasarNP#7:

model, wavegrid = load_model(modelfilename)
blat = load_desi_coadd(coaddfilename, out_grid=wavegrid)

Previously there were hardcoded assumptions about the model wavelength grid; now the model files themselves specify what wavegrid was used.