Remove assets for removed django-companies element

The SCSS and images removed here predate even 73d29b7, so it's hard to
tell when they were added. Searching shows that the images, class, and
mixin are no longer used. Plus, the Rdio service was discontinued at the
end of 2015, so that's a hint.

djangoproject/scss/_style.scss

@@ -833,53 +833,6 @@ blockquote {
                 background: darken($green-medium, 2%);
             }
         }
-
-        .django-companies {
-            max-width: 750px;
-            margin: 0 auto;
-            list-style: none;
-            padding: 0 0 0 30px;
-
-            li {
-                width: 144px;
-                display: inline-block;
-                text-indent: -1000px;
-                overflow: hidden;
-                margin: 0;
-
-                a {
-                    display: block;
-                    height: 46px;
-                    background-position: center;
-                    background-repeat: no-repeat;
-
-                    &.company-mozilla {
-                        @include svg-image(company-mozilla);
-                        background-position: center 7px;
-                    }
-
-                    &.company-pinterest {
-                        @include svg-image(company-pinterest);
-                        background-position: center 7px;
-                    }
-
-                    &.company-theguardian {
-                        @include svg-image(company-theguardian);
-                        background-position: center 14px;
-                    }
-
-                    &.company-instagram {
-                        @include svg-image(company-instagram);
-                        background-position: center 10px;
-                    }
-
-                    &.company-rdio {
-                        @include svg-image(company-rdio);
-                        background-position: 15px 6px;
-                    }
-                }
-            }
-        }
     }
 }
 

djangoproject/scss/_utils.scss

@@ -135,14 +135,6 @@ html {
     }
 }
 
-@mixin svg-image ($name ) {
-    background-image: url('../img/' +$name+'.png');
-
-    .mdzr-svg & {
-        background-image: url('../img/' +$name+'.svg');
-    }
-}
-
 // Green Link Mixin
 @mixin link-green {
     color: var(--link-color);