Remove Vega 3 extension as it will be moved to JupyterLab

Author: domoritz

README.md

@@ -13,7 +13,6 @@ This is a [monorepo](https://github.com/lerna/lerna#what-does-a-lerna-repo-look-
 | [katex-extension](packages/katex-extension) | N/A | N/A |
 | [plotly-extension](packages/plotly-extension) | `application/vnd.plotly.v1+json` | `.plotly`, `.plotly.json` |
 | [vega2-extension](packages/vega2-extension) | `application/vnd.vega.v2+json`, `application/vnd.vegalite.v1+json`| `.vg`, `.vl`, `.vg.json`, `.vl.json`, `.vega`, `.vegalite` |
-| [vega3-extension](packages/vega3-extension) | `application/vnd.vega.v3+json`, `application/vnd.vegalite.v2+json`| `.vg`, `.vl`, `.vg.json`, `.vl.json`, `.vega`, `.vegalite` |
 
 ## Install
 
@@ -22,7 +21,6 @@ This is a [monorepo](https://github.com/lerna/lerna#what-does-a-lerna-repo-look-
 * katex-extension: `jupyter labextension install @jupyterlab/katex-extension`
 * plotly-extension: `jupyter labextension install @jupyterlab/plotly-extension`
 * vega2-extension: `jupyter labextension install @jupyterlab/vega2-extension`
-* vega3-extension: `jupyter labextension install @jupyterlab/vega3-extension`
 
 ## Contributing