Strip description encoding channel for improved render performance

[jonmmease]

In the course of doing some benchmarking of pan and zoom on large scatterplots, I found that the description encoding that Vega-Lite adds by default has a significant performance penalty.

Here are some chrome profile flamegraphs for a 40k row version of the Vega-Lite pan/zoom example at https://vega.github.io/vega-lite/examples/selection_translate_scatterplot_drag.html

Default canvas renderer

With description and ariaRoleDescription encoding channels removed.

These encoding channels have no effect when used with the canvas renderer, which is what Vega-Altair uses.