Re #6191. Remove MatrixWorkspace::getWorkspaceIndexToDetectorIDMap.

It is redundant alongside the ISpectrum objects and was only being used
in tests and in one place in the ISIS SANS GUI (for which I've put in what
I believe to be equivalent code, though there's no test to verify it (too
much logic in the GUI layer).
This also resolves part of #6198.
Re #6191. Remove MatrixWorkspace::getWorkspaceIndexToSpectrumMap.

It is redundant (just use ISpectrum) and was not being used anywhere.
With this commit, #6198 is half way done.
Re #6191. Remove unnecessary calls to generateSpectraMap().
Re #6191. Remove another unused method.
Re #6191. Doxygen warning fixes.
Merge branch 'feature/6191_remove_spectradetectormap' into develop