Re #6191. rebuildSpectraMapping doesn't touch the SpectraDetectorMap.
Re #6191. Reapply the removal of the SpectraDetectorMap.

Revert "Revert "Re #6191. Don't use the SpectraDetectorMap.""
This reverts commit f6eb0e0c87acd701f67ad170ed8312a2089e5da6.