Remove min--moz-device-pixel-ratio

Bootstrap v4 no longer supports the old versions of Firefox affected by this typo bug.

scss/mixins/_image.scss

@@ -16,15 +16,13 @@
 
 // Retina image
 //
-// Short retina mixin for setting background-image and -size. Note that the
-// spelling of `min--moz-device-pixel-ratio` is intentional.
+// 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}");
 
   @media
   only screen and (-webkit-min-device-pixel-ratio: 2),
-  only screen and (   min--moz-device-pixel-ratio: 2),
   only screen and (     -o-min-device-pixel-ratio: 2/1),
   only screen and (        min-device-pixel-ratio: 2),
   only screen and (                min-resolution: 192dpi),