Remove nested breakpoint in fullscreen-mode CSS (WordPress#21496)

A `break-medium` mixin in `/packages/interface/src/components/fullscreen-mode/style.scss` is nested within the previous `break-medium`. 

So it currently compiles with extra media queries and an unused value of 46px.

```
@media (min-width:782px){body.js.is-fullscreen-mode{margin-top:-46px;height:calc(100% + 46px)}}@media (min-width:782px) and (min-width:782px){body.js.is-fullscreen-mode{margin-top:-32px;height:calc(100% + 32px)}}@media (min-width:782px){
```

packages/interface/src/components/fullscreen-mode/style.scss

@@ -3,12 +3,8 @@ body.js.is-fullscreen-mode {
 	@include break-medium {
 		// Reset the html.wp-topbar padding.
 		// Because this uses negative margins, we have to compensate for the height.
-		margin-top: -$admin-bar-height-big;
-		height: calc(100% + #{ $admin-bar-height-big });
-		@include break-medium() {
-			margin-top: -$admin-bar-height;
-			height: calc(100% + #{ $admin-bar-height });
-		}
+		margin-top: -$admin-bar-height;
+		height: calc(100% + #{ $admin-bar-height });
 
 		#adminmenumain,
 		#wpadminbar {