Reduce HGC rechit memory footprint

Remove a merging module that is un-necessary, should reduce HGC rechit memory footprint by 2x.

Trying to get a working rechit creator that can service all three HGC subcomponents at once, removing the need for a merging module. For some reason it will not let any rechits pass. I working on a fix since this is necessary to keep the memory use down once we start collection rechits in the whole detector at once.

This is a decent patch for now since the subcomponents are clustered separately in this version of HGC RECO.

Last commit should help reduce the retained memory by quite a bit.

@pfs @vandreev11