Trying a new LCG layer for Sherpa2/3 and deleting some problemetic Sherpa2 code.
@fwinkl @akraszna
This MR goes into main, which has Sherpa3. But the MR was tested with b_ATLAS_25, which has Sherpa2. If needed, this can be backported to 23.0, but it will require layer b_ATLAS_25+.
@nstyles