Fix minor error with handlerStack in content script

There is minor error `Uncaught TypeError: Object #<Object> has no method 'remove'` thrown by 
```coffee
      handlerStack.push({
        keyup: (event) =>
          # ...
          @remove() # this line
      })
```
in `link_hints.coffee` content script. To reproduce, try press `f` and then `Shift` (yep, just `Shift`) on any page with enabled Vimium built from current `master` branch.