Remove unsafe terser optimizations (#6889)

This optimizer feature was causing  the native Symbol implementation to
be replaced for some reason. This in turn causes the Closure polyfills
code to break inside validator_wasm.js

Author: erwinmombay

playground/webpack.config.js

@@ -44,7 +44,6 @@ module.exports = (env, argv) => {
             ecma: '2015',
             compress: {
               defaults: true,
-              unsafe: true,
             },
           },
         }),