Drop deprecated img-retina mixin (#28459)

scss/mixins/_image.scss

@@ -14,23 +14,3 @@
   // when setting a width and height attribute on the img element.
   height: auto;
 }
-
-
-// Retina image
-//
-// Short retina mixin for setting background-image and -size.
-
-@mixin img-retina($file-1x, $file-2x, $width-1x, $height-1x) {
-  background-image: url($file-1x);
-
-  // Autoprefixer takes care of adding -webkit-min-device-pixel-ratio and -o-min-device-pixel-ratio,
-  // but doesn't convert dppx=>dpi.
-  // There's no such thing as unprefixed min-device-pixel-ratio since it's nonstandard.
-  // Compatibility info: https://caniuse.com/#feat=css-media-resolution
-  @media only screen and (min-resolution: 192dpi), // IE9-11 don't support dppx
-    only screen and (min-resolution: 2dppx) { // Standardized
-    background-image: url($file-2x);
-    background-size: $width-1x $height-1x;
-  }
-  @include deprecate("`img-retina()`", "v4.3.0", "v5");
-}