Re #6191. Remove check that's no longer valid.
Re #6191. Change getDetectorIDToWorkspaceIndexMap to use ISpectrum.

Instead of spectraDetectorMap.
Re #6191. Remove unnecessary calls to generateSpectraMap().
Re #6191. Remove unnecessary call to updateSpectraUsingMap().
Re #6191. Remove unneeded code.
Merge branch 'feature/6191_remove_spectradetectormap' into develop