Remove therubyracer support
The bundler group for therubyracer (based on libv8) can be removed as there is now a development/build dependency on nodejs via #12364 and use of npm/webpack. execjs will automatically use nodejs when is installed, so therubyracer
We no longer have any operating systems relying on therubyracer for asset precompilation either, they all have nodejs available. This will save time building and installing gems.