Prevent duplicate signals from being generated for top-level LayerModel. (#5350)
Prevent duplicate signals from being generated for top-level LayerModel. (#5350)

When a LayerModel is at the top-level, top-level signals and unit model
signals all get flattened into the same array. As a result, we would
have bound projection signals alongside their "push: outer" siblings.
Although technically incorrect, this approach worked fine up until
Vega 5.5, when a change to the dataflow was introduced.
vega/vega-lite,vega/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,vega/vega-lite,uwdata/vega-lite,uwdata/vega-lite