remove Encoding.toggleFilterNullO, which should just be a part of `vega-lite-ui`
remove Encoding.toggleFilterNullO, which should just be a part of `vega-lite-ui`

Address part of #598

ellisonbg/vega-lite,vega/vega-lite,vega/vega-lite,uwdata/vega-lite,uwdata/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,vega/vega-lite,ellisonbg/vega-lite,ellisonbg/vega-lite