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.
Super FX: Remove massive amounts of indirection starting in FxEmulate.

* FxEmulate now uses fx_run directly.
* Pointers to function pointers that were formerly selected according to whether Super FX breakpoints are enabled are replaced with references to the actual array. This allows the linker to insert constant addresses.
* Super FX breakpoints are now unsupported.