code simplification for mandelbrot_task.ispc

I simplified the kernel, which basically uses 1D tasking as a subset of 2D tasking (with ntaskx = 1). On my Haswell MacbookAir, I see about 1-2% difference between 2D and 1D in both modified and unmodified kernels.  If there is performance hit between modified and unmodified 1D version, I can't see it.

Ilia, would we like to keep this change instead, at least if, after checking on your side, this simplification is as fast as before..

Cheers,
 Evghenii