Changed `@include background-size()` to `background-size`

Bourbon's background-size mixin has been deprecated, and they recommend to use the
official spec.

common/lib/xmodule/xmodule/css/video/display.scss

@@ -64,7 +64,7 @@ div.video {
 
         a.ui-slider-handle {
           background: $pink url(../images/slider-handle.png) center center no-repeat;
-          @include background-size(50%);
+          background-size: 50%;
           border: 1px solid darken($pink, 20%);
           border-radius: 15px;
           box-shadow: inset 0 1px 0 lighten($pink, 10%);
@@ -320,7 +320,7 @@ div.video {
 
               a.ui-slider-handle {
                 background: $pink url(../images/slider-handle.png) center center no-repeat;
-                @include background-size(50%);
+                background-size: 50%;
                 border: 1px solid darken($pink, 20%);
                 border-radius: 15px;
                 box-shadow: inset 0 1px 0 lighten($pink, 10%);

common/lib/xmodule/xmodule/css/videoalpha/display.scss

@@ -64,7 +64,7 @@ div.video {
 
         a.ui-slider-handle {
           background: $pink url(../images/slider-handle.png) center center no-repeat;
-          @include background-size(50%);
+          background-size: 50%;
           border: 1px solid darken($pink, 20%);
           border-radius: 15px;
           box-shadow: inset 0 1px 0 lighten($pink, 10%);
@@ -320,7 +320,7 @@ div.video {
 
               a.ui-slider-handle {
                 background: $pink url(../images/slider-handle.png) center center no-repeat;
-                @include background-size(50%);
+                background-size: 50%;
                 border: 1px solid darken($pink, 20%);
                 border-radius: 15px;
                 box-shadow: inset 0 1px 0 lighten($pink, 10%);

lms/static/sass/multicourse/_home.scss

@@ -102,7 +102,7 @@
         overflow: hidden;
         position: relative;
         background: url($video-thumb-url) center no-repeat;
-        @include background-size(cover);
+        background-size: cover;
 
         .play-intro {
           @include background-image(linear-gradient(-90deg, rgba(0,0,0, 0.65), rgba(0,0,0, 0.75)));