Revert "Images are responsive by default"

This reverts commit d29f851e82.

Making <img>s responsive by-default without opt-in can severely break
third-party widgets such as Google Maps.
This was an acknowledged problem in Bootstrap v2
(see https://github.com/twbs/bootstrap/issues/1506 ) and was fixed in v3
by requiring the .img-responsive class for explicit opt-in
(see 09cdee2f03).
The situation hasn't really changed since then. The responsive-by-default
approach hasn't become any more suitable in the intervening time.

So let's avoid having this regress in v4. :-)

scss/_carousel.scss

@@ -21,7 +21,7 @@
     // Account for jankitude on images
     > img,
     > a > img {
-      // @extend .img-responsive;
+      @extend .img-responsive;
       line-height: 1;
     }