carousel.js: remove unneeded variable assignment.

`slideIndex` is assigned to the same value a few lines above.

js/carousel.js

@@ -194,7 +194,7 @@
 
     Plugin.call($target, options)
 
-    if (slideIndex = $this.attr('data-slide-to')) {
+    if (slideIndex) {
       $target.data('bs.carousel').to(slideIndex)
     }