freedreno/ir3: move rasterflat handling out of ir3
Remove the rasterflat shader key. a4xx/a5xx/a6xx will now do the same thing as a3xx (bary.f with flat interpolation set by the interp regs when flat shading is enabled).
Remove the rasterflat shader key. a4xx/a5xx/a6xx will now do the same thing as a3xx (bary.f with flat interpolation set by the interp regs when flat shading is enabled).