fxemu.cpp: Do not use GSU.pfRpix and GSU.pfPlot to pass SuperFX plotter functions around.

Based on https://github.com/libretro/snes9x-next/commit/e9c304af88fc5f69104eee9e19e6be0a9e3e9d6e by TwinAphex51224; some additional code modifications are removed.