Surface.h rm thread unsafe version of setMaterialLayer, move the const_cast to the one remaining code that actually needs them

Christos Anastopoulosrequested to merge
ATLAS-EGamma/athena:checkout_rm_MT_unsafe_version_of_setMaterialLayer into master

Surface.h rm thread unsafe version of setMaterialLayer, move the const_cast to the one remaining code that actually needs them

Mentioning @sroe

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