[CS2] Remove unnecessary utility helper functions (#4526)

* Uncomment module test that was waiting for classes to be supported

* Rely on native indexOf

* Replace `bind` helper with native `bind` (no shortcut necessary)

* Update output

* Update modules class tests

* Remove helper for prototype extends

* Update docs to reflect removed extends operator

* Add shortcut for splice, like we have for slice

lib/coffeescript/grammar.js

@@ -783,8 +783,6 @@
         return new Assign($1, $4, $2);
       }), o('SimpleAssignable COMPOUND_ASSIGN TERMINATOR Expression', function() {
         return new Assign($1, $4, $2);
-      }), o('SimpleAssignable EXTENDS Expression', function() {
-        return new Extends($1, $3);
       })
     ]
   };