Re #6191. Remove SpectraDetectorMap member from MatrixWorkspace.

And the methods that used it (that were not called anywhere any longer).
Re #6191. Remove the SpectraDetectorMap class.
Re #6191. Remove ISpectraDetectorMap & OneToOneSpectraDetectorMap.
Re #6191. Remove last few references to the SpectraDetectorMap.

Mostly in comments, plus one forward declaration.