Extract responsive embed nested classes

I think this is the more common coding convention across Bootstrap.

Extract nested classes as first-class citizens and assume HTML would use both.

less/responsive-embed.less

@@ -22,14 +22,14 @@
     width: 100%;
     border: 0;
   }
+}
 
-  // Modifier class for 16:9 aspect ratio
-  &.embed-responsive-16by9 {
+// Modifier class for 16:9 aspect ratio
+.embed-responsive-16by9 {
   padding-bottom: 56.25%;
-  }
+}
 
-  // Modifier class for 4:3 aspect ratio
-  &.embed-responsive-4by3 {
+// Modifier class for 4:3 aspect ratio
+.embed-responsive-4by3 {
   padding-bottom: 75%;
-  }
 }