remove assert in scaleAlphaChannel

If you open up examples/filter/filter.html, the shadow filter is broken (the color doesn't run either but that's because a JS error is thrown from the shadow filter).

The problem is that the filter driver is calling `dropShadowFilter` with a `strength` of .5, which is being passed to `scaleAlphaChannel` in filter.js. `isNumeric` actually enforce integral types, so it fails, but I bet it used to accept all numeric types and this assert is leftover.