Remove background behind images in markdown-body (#2691)

* Remove background behind images in markdown-body

* Create shiny-students-juggle.md

---------

Co-authored-by: Jon Rohan <[email protected]>

Author: dylanatsmith

.changeset/shiny-students-juggle.md

@@ -0,0 +1,5 @@
+---
+"@primer/css": minor
+---
+
+Remove background behind images in markdown-body

src/markdown/images.scss

@@ -8,7 +8,6 @@
     // because we put padding on the images to hide header lines, and some people
     // specify the width of their images in their markdown.
     box-sizing: content-box;
-    background-color: var(--bgColor-default, var(--color-canvas-default));
 
     &[align='right'] {
       // stylelint-disable-next-line primer/spacing