Remove outdated eco package from bench

The `eco`-templates (Embedded CoffeeScript templates)
had their last update over 10 years ago, so we can
remove this dependency from our benchmark.

tests/bench/templates/string.js

@@ -2,6 +2,5 @@ module.exports = {
   context: {},
   handlebars: 'Hello world',
   dust: 'Hello world',
-  mustache: 'Hello world',
-  eco: 'Hello world'
+  mustache: 'Hello world'
 };