Remove inappropriate role="listbox" from carousels

docs/components/carousel.md

@@ -28,7 +28,7 @@ Here's a carousel with slides only. Note the presence of the `.d-block` and `.im
 
 {% example html %}
 <div id="carouselExampleSlidesOnly" class="carousel slide" data-ride="carousel">
-  <div class="carousel-inner" role="listbox">
+  <div class="carousel-inner">
     <div class="carousel-item active">
       <img class="d-block w-100" data-src="holder.js/800x400?auto=yes&bg=777&fg=555&text=First slide" alt="First slide">
     </div>
@@ -48,7 +48,7 @@ Adding in the previous and next controls:
 
 {% example html %}
 <div id="carouselExampleControls" class="carousel slide" data-ride="carousel">
-  <div class="carousel-inner" role="listbox">
+  <div class="carousel-inner">
     <div class="carousel-item active">
       <img class="d-block w-100" data-src="holder.js/800x400?auto=yes&bg=777&fg=555&text=First slide" alt="First slide">
     </div>
@@ -81,7 +81,7 @@ You can also add the indicators to the carousel, alongside the controls, too.
     <li data-target="#carouselExampleIndicators" data-slide-to="1"></li>
     <li data-target="#carouselExampleIndicators" data-slide-to="2"></li>
   </ol>
-  <div class="carousel-inner" role="listbox">
+  <div class="carousel-inner">
     <div class="carousel-item active">
       <img class="d-block w-100" data-src="holder.js/800x400?auto=yes&bg=777&fg=555&text=First slide" alt="First slide">
     </div>
@@ -120,7 +120,7 @@ Add captions to your slides easily with the `.carousel-caption` element within a
       <li data-target="#carouselExampleCaptions" data-slide-to="1"></li>
       <li data-target="#carouselExampleCaptions" data-slide-to="2"></li>
     </ol>
-    <div class="carousel-inner" role="listbox">
+    <div class="carousel-inner">
       <div class="carousel-item active">
         <img class="d-block w-100" data-src="holder.js/800x400?auto=yes&bg=777&fg=555&text=First slide" alt="First slide">
         <div class="carousel-caption d-none d-md-block">