Remove clamp

I think there was consensus to remove clamp from float32x4 / float64x2, as it can be emulated with a min and a max.

```
// For float32x4
function clamp(v, low, high) {
 return SIMD.float32x4.max(SIMD.float32x4.min(v, high), low);
}
```

If that's still the case, could we just remove it from the spec / polyfill?