Remove long-unused internal plugin system

We are trying to update Express to fix a vulnerability.

We would have to update the plugins as well, but since we are no longer
using the plugin system, we can just delete it instead.

ci/build/build-release.sh

@@ -44,10 +44,6 @@ bundle_code_server() {
   rsync src/browser/pages/*.css "$RELEASE_PATH/src/browser/pages"
   rsync src/browser/robots.txt "$RELEASE_PATH/src/browser"
 
-  # Add typings for plugins
-  mkdir -p "$RELEASE_PATH/typings"
-  rsync typings/pluginapi.d.ts "$RELEASE_PATH/typings"
-
   # Adds the commit to package.json
   jq --slurp '(.[0] | del(.scripts,.jest,.devDependencies)) * .[1]' package.json <(
     cat << EOF