Reduce the number of files in MG gridpacks

Baptiste Ravinarequested to merge
ravinab/athena:21.6-smaller-MG-gridpacks into 21.6

As discussed in ATLMCPROD-8537, there are a large number of files in the LO MadGraph gridpacks that we are currently keeping but don't actually need. This MR removes them, keeping only the integration grids and the results.dat.

For NLO gridpacks, also remove the events.lhe and optional events.lhe.rwgt files created for the original gridpack run.

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