Reduce local variables usage in CoffeeScript

Local variables are one of the things CoffeeScript doesn't compile to
modern JavaScript automatically: it uses `var` instead of `const` or
`let`.

Besides, using `$this = $(this)` is usually done to reference the
current object in another function where the current object is a
different one. Here we were using it with no clear purpose.

app/assets/javascripts/checkbox_toggle.js.coffee

@@ -4,9 +4,8 @@ App.CheckboxToggle =
 
   initialize: ->
     $("[data-checkbox-toggle]").on "change", ->
-      $this = $(this)
-      $target = $($this.data("checkbox-toggle"))
-      if $this.is(":checked")
+      $target = $($(this).data("checkbox-toggle"))
+      if $(this).is(":checked")
         $target.show()
       else
         $target.hide()