[Metaballs] Remove sampling kernel from Metal file

Metaballs/Shaders.metal

@@ -14,13 +14,6 @@ typedef struct {
     float radius;
 } Ball;
 
-kernel void
-sampleFieldKernel(const device Ball* metaballs  [[buffer(0)]],
-                  device float* samples         [[buffer(1)]],
-                  uint2 gid                     [[thread_position_in_grid]])
-{
-    // TODO: Compute a sample for this pixel given the field data, and write it to the out texture.
-}
 
 typedef struct {
     float2 position;