Generally if you where replacing just the bearings over to Greens there's no need to remove the thrust button, but when you retrofit an aftermarket rear disc kit, the spacers (if any) supplied with the kit to replace/restore the thickness of the drum brake plates discarded, are not the same thickness, so this moves the axles inward loading the thrust button excessively, or may even prevent you from seating the axles/bearings/flange...

So you can either remove the thrust button from the 3rd member, or machine an axle/flange spacer that is of the same thickness of the brake backing plate your discarding if the spacer supplied in the kit is not adequate...

I myself prefer to retain the thrust button and machine a set of spacers as required to position the axles in their "stock" location if needed, it's going to depend on the kit supplier, but measure all critical distances/specs of where the axles sit in there stock configuration before letting the parts fly off

Mike