No background for code in portability snippets

This better matches the appearance of this kind of snippet in the full
item view and is less jarring to read due to repeated
foreground-background changes.

Author: nagisa

src/librustdoc/html/static/themes/ayu.css

@@ -266,7 +266,7 @@ a {
 
 .stab.portability > code {
 	color: #e6e1cf;
-	background-color: transparent;
+	background: none;
 }
 
 #help > div {

src/librustdoc/html/static/themes/dark.css

@@ -222,10 +222,7 @@ a.test-arrow {
 .stab.unstable { background: #FFF5D6; border-color: #FFC600; color: #2f2f2f; }
 .stab.deprecated { background: #F3DFFF; border-color: #7F0087; color: #2f2f2f; }
 .stab.portability { background: #C4ECFF; border-color: #7BA5DB; color: #2f2f2f; }
-
-.stab.portability > code {
-	color: #ddd;
-}
+.stab.portability > code { background: none; }
 
 #help > div {
 	background: #4d4d4d;

src/librustdoc/html/static/themes/light.css

@@ -220,10 +220,7 @@ a.test-arrow {
 .stab.unstable { background: #FFF5D6; border-color: #FFC600; }
 .stab.deprecated { background: #F3DFFF; border-color: #7F0087; }
 .stab.portability { background: #C4ECFF; border-color: #7BA5DB; }
-
-.stab.portability > code {
-	color: #000;
-}
+.stab.portability > code { background: none; }
 
 #help > div {
 	background: #e9e9e9;