remove custom highlighting

maybe temporarily. seems inconsistent.
closes statamic#1545
closes statamic#1548

Author: jasonvarga

resources/css/content.css

@@ -1,11 +1,6 @@
 /* Main Content */
 /* -------------------------------------------------- */
 
-::-moz-selection,
-::selection {
-    @apply bg-yellow;
-}
-
 article, .markdown {
     background: white;
     @apply text-gray-darkest leading-relaxed;