Remove the legacy APR21 setups Refs #2934 #2936
Alternative to !237 (closed) for clearing #2936
To be decided by Physics groups and @v.friese what should be the solution picked (one of the 2 MRs HAS to be picked)
May also be extended to remove also the DEC21 setups depending on decision in #2935
Of interest to @e.clerkin @v.friese @i.selyuzhenkov