Turn off uglify caching in production builds.

This seems to be causing an error in Appveyor (such as https://ci.appveyor.com/project/jupyterlab/jupyterlab/build/1.0.7521/job/cc42qxscjdpm5t0n - python 3.6 has an error removing one of the cached files when cleaning up a temporary directory), so we’ll revert to the old non-cached behavior for now.

dev_mode/webpack.prod.config.js

@@ -8,7 +8,6 @@ module.exports = merge(common, {
   optimization: {
     minimizer: [
       new UglifyJsPlugin({
-        cache: true,
         parallel: true,
         sourceMap: true,
         uglifyOptions: {

jupyterlab/staging/webpack.prod.config.js

@@ -8,7 +8,6 @@ module.exports = merge(common, {
   optimization: {
     minimizer: [
       new UglifyJsPlugin({
-        cache: true,
         parallel: true,
         sourceMap: true,
         uglifyOptions: {