Remove `thumbnail-transition` from `.img-thumbnail`.

We don't change anything so the transition is basically unused.

scss/_images.scss

@@ -16,7 +16,6 @@
   background-color: $thumbnail-bg;
   border: $thumbnail-border-width solid $thumbnail-border-color;
   @include border-radius($thumbnail-border-radius);
-  @include transition($thumbnail-transition);
   @include box-shadow($thumbnail-box-shadow);
 
   // Keep them at most 100% wide

scss/_variables.scss

@@ -761,7 +761,6 @@ $thumbnail-border-width:            $border-width !default;
 $thumbnail-border-color:            #ddd !default;
 $thumbnail-border-radius:           $border-radius !default;
 $thumbnail-box-shadow:              0 1px 2px rgba($black,.075) !default;
-$thumbnail-transition:              all .2s ease-in-out !default;
 
 
 // Figures