Re #6191. Remove SpectraDetectorMap use from LoadMuonNexus2.

It was just setting up a 1-1 mapping, which is what you get by default
anyway.
Re #6191. Remove use of SpectraDetectorMap.
Re #6191. Remove SpectraDetectorMap from comment.
Re #6191. Use new setDetectorIDs method.
Revert "Re #6191. Don't use the SpectraDetectorMap."

This breaks some system tests. Looks like I'd better work
from the other end or the inside out instead.

This reverts commit 3407c5df4a95053d8c2ee2e227e2219082836bd0.